convert <file>:<line> into github permalink

ghpath.sh

#!/bin/sh
#
# Turn a file/line combination into a link to github.com.
# I use this from vim with:
#
#   command! Link :exec "!ghpath %:p " . line(".")
#
# Running ":Link" anywhere will get you a link to the current file and
# line number. Remember to drop this as "ghpath" into your PATH, and
# to turn on the executable bit.

file=$1
line=$2

cd "$(dirname "$file")" || exit 1

url=$(git config remote.origin.url)
url=${url%.git}
if ! repo=$(expr "$url" : "https://github.com/\(.*\)") &&
   ! repo=$(expr "$url" : "git@github.com:\(.*\)") &&
   ! repo=$(expr "$url" : "git://github.com/\(.*\)")
then
	echo >&2 "fatal: origin repo is not at github: $url"
	exit 1
fi

# We can't use our commit sha directly, because we
# might be on a commit that has not yet been pushed.
# But finding the merge-base of our current HEAD
# and the remote refs is a good guess. If the remote
# does have our commit, we will use it. And otherwise, we
# should walk back to the nearest fork point (i.e.,
# what we started building on from upstream).
commit=$(git merge-base HEAD \
	$(git for-each-ref refs/remotes/origin --format='%(objectname)'))
path="$(git rev-parse --show-prefix)$(basename "$file")"

if tag=$(git describe --exact-match "$commit" 2>/dev/null); then
	commit=$tag
fi

# XXX should probably urlencode
echo "https://github.com/$repo/blob/$commit/$path#L$line"