problem with git and GitHub fetching pull requests

.git--config

[core]
	repositoryformatversion = 0
	filemode = true
	bare = false
	logallrefupdates = true
	ignorecase = true
	precomposeunicode = true
[remote "kevin"]
	url = git@github.com:killerswan/ponyc.git
	fetch = +refs/heads/*:refs/remotes/kevin/*
[branch "master"]
	remote = kevin
	merge = refs/heads/master
[remote "upstream"]
	url = git@github.com:ponylang/ponyc.git
	fetch = +refs/heads/*:refs/remotes/upstream/*
	fetch = +refs/pull/*:refs/remotes/upstream/pr/*

logs

code $ git clone git@github.com:killerswan/ponyc.git --origin kevin
Cloning into 'ponyc'...
remote: Counting objects: 53628, done.
remote: Compressing objects: 100% (32/32), done.
remote: Total 53628 (delta 10), reused 0 (delta 0), pack-reused 53596
Receiving objects: 100% (53628/53628), 24.29 MiB | 7.45 MiB/s, done.
Resolving deltas: 100% (33610/33610), done.
code $ cd ponyc
ponyc $ git remote -v && git remote add upstream git@github.com:ponylang/ponyc.git
...
ponyc $ git remote -v
kevin	git@github.com:killerswan/ponyc.git (fetch)
kevin	git@github.com:killerswan/ponyc.git (push)
upstream	git@github.com:ponylang/ponyc.git (fetch)
upstream	git@github.com:ponylang/ponyc.git (push)
ponyc $ 
ponyc $ 
ponyc $ git fetch kevin && git fetch upstream
remote: Counting objects: 175159, done.
remote: Total 175159 (delta 39327), reused 39327 (delta 39327), pack-reused 135832
Receiving objects: 100% (175159/175159), 113.02 MiB | 10.19 MiB/s, done.
Resolving deltas: 100% (89080/89080), completed with 343 local objects.
From github.com:ponylang/ponyc
 * [new branch]          MinGW64                         -> upstream/MinGW64
 * [new branch]          adaptive-per-actor-scheduler-batch-sizes -> upstream/adaptive-per-actor-scheduler-batch-sizes
 * [new branch]          array-as-sequence               -> upstream/array-as-sequence
 * [new branch]          assume-nominal                  -> upstream/assume-nominal
 * [new branch]          backpressure                    -> upstream/backpressure
 * [new branch]          baseline-prebuilt-binaries      -> upstream/baseline-prebuilt-binaries
 * [new branch]          enhancement/autorecover-rhs     -> upstream/enhancement/autorecover-rhs
 * [new branch]          enhancement/files-doc-string    -> upstream/enhancement/files-doc-string
 * [new branch]          feature/exhaustive-match        -> upstream/feature/exhaustive-match
 * [new branch]          fix/gencap-violation            -> upstream/fix/gencap-violation
 * [new branch]          fix/reader-append-byteseq       -> upstream/fix/reader-append-byteseq
 * [new branch]          get-or-else-perf                -> upstream/get-or-else-perf
 * [new branch]          gh-pages                        -> upstream/gh-pages
 * [new branch]          how-to-pr                       -> upstream/how-to-pr
 * [new branch]          llvm-3.9.1                      -> upstream/llvm-3.9.1
 * [new branch]          map-insert_if_absent            -> upstream/map-insert_if_absent
 * [new branch]          master                          -> upstream/master
 * [new branch]          net-address-equality            -> upstream/net-address-equality
 * [new branch]          pr/1227                         -> upstream/pr/1227
 * [new branch]          refactor/add-refer-pass         -> upstream/refactor/add-refer-pass
 * [new branch]          refactor/flatten-union-types-in-parser -> upstream/refactor/flatten-union-types-in-parser
 * [new branch]          refactor/reference-pass         -> upstream/refactor/reference-pass
 * [new branch]          refactor/status-pass            -> upstream/refactor/status-pass
 * [new branch]          release                         -> upstream/release
 * [new branch]          remove-union-check              -> upstream/remove-union-check
 * [new branch]          rfc-41                          -> upstream/rfc-41
 * [new branch]          sean-runtime-backpressure       -> upstream/sean-runtime-backpressure
 * [new branch]          skip-ci-contributing            -> upstream/skip-ci-contributing
 * [new branch]          skip-unneeded-ci-on-release     -> upstream/skip-unneeded-ci-on-release
 * [new branch]          tcp-connection-dont-double-resubscribe -> upstream/tcp-connection-dont-double-resubscribe
 * [new branch]          tcp-one-shot-linux              -> upstream/tcp-one-shot-linux
 * [new branch]          trace-when-freezing-on-send     -> upstream/trace-when-freezing-on-send
 * [new branch]          update-api-docs-only-on-release -> upstream/update-api-docs-only-on-release
 * [new branch]          update-osx-from-source-instructions -> upstream/update-osx-from-source-instructions
 * [new tag]             0.12.2                          -> 0.12.2
 * [new tag]             0.3.0                           -> 0.3.0
 * [new tag]             0.3.1                           -> 0.3.1
 * [new tag]             0.3.2                           -> 0.3.2
 * [new tag]             0.3.3                           -> 0.3.3
 * [new tag]             0.4.0                           -> 0.4.0
 * [new tag]             0.10.0                          -> 0.10.0
 * [new tag]             0.11.0                          -> 0.11.0
 * [new tag]             0.11.1                          -> 0.11.1
 * [new tag]             0.11.2                          -> 0.11.2
 * [new tag]             0.11.3                          -> 0.11.3
 * [new tag]             0.11.4                          -> 0.11.4
 * [new tag]             0.12.0                          -> 0.12.0
 * [new tag]             0.12.1                          -> 0.12.1
 * [new tag]             0.5.0                           -> 0.5.0
 * [new tag]             0.5.1                           -> 0.5.1
 * [new tag]             0.6.0                           -> 0.6.0
 * [new tag]             0.7.0                           -> 0.7.0
 * [new tag]             0.8.0                           -> 0.8.0
 * [new tag]             0.9.0                           -> 0.9.0
ponyc $ 
ponyc $ 
ponyc $ 
ponyc $ gvim .git/config 
ponyc $ git fetch upstream
remote: Counting objects: 4910, done.
remote: Compressing objects: 100% (47/47), done.
remote: Total 4910 (delta 3025), reused 3008 (delta 3008), pack-reused 1855
Receiving objects: 100% (4910/4910), 1.05 MiB | 0 bytes/s, done.
Resolving deltas: 100% (3234/3234), completed with 1458 local objects.
From github.com:ponylang/ponyc
 * [new ref]             refs/pull/1001/head  -> upstream/pr/1001/head
 * [new ref]             refs/pull/1002/head  -> upstream/pr/1002/head
 * [new ref]             refs/pull/1003/head  -> upstream/pr/1003/head
...
 * [new ref]             refs/pull/1224/head  -> upstream/pr/1224/head
 * [new ref]             refs/pull/1224/merge -> upstream/pr/1224/merge
 * [new ref]             refs/pull/1226/head  -> upstream/pr/1226/head
 * [new ref]             refs/pull/1226/merge -> upstream/pr/1226/merge
error: cannot lock ref 'refs/remotes/upstream/pr/1227/head': 'refs/remotes/upstream/pr/1227' exists; cannot create 'refs/remotes/upstream/pr/1227/head'
 ! [new ref]             refs/pull/1227/head  -> upstream/pr/1227/head  (unable to update local ref)
 * [new ref]             refs/pull/1228/head  -> upstream/pr/1228/head
 * [new ref]             refs/pull/1229/head  -> upstream/pr/1229/head
...
 * [new ref]             refs/pull/998/head   -> upstream/pr/998/head
 * [new ref]             refs/pull/999/head   -> upstream/pr/999/head
error: some local refs could not be updated; try running
 'git remote prune upstream' to remove any old, conflicting branches

On a totally new clone, I add this to the .git/config:

fetch = +refs/pull/*:refs/remotes/upstream/pr/*

And then I can git fetch upstream to get the pull request head and merge branches from GitHub.... Except there's this problem locking the ref for 1227/head. Both in my old copy of the repo, and this new clone! 🙀