Extract subdirectory in git, then slim down the history

extract_subdir_slim_history.txt

$ git clone blahblahblah                # <-- fresh clone of the repo
$ du -sh .
768M                                    # <-- holy macaroni, that's huge

$ git filter-branch --subdirectory-filter path/to/subdir --prune-empty --tag-name-filter cat -- --all     # <-- Actual filter

    ...

### Now we'll delete all references to those unchanged refs
$ git reflog expire --expire=now --all  # <-- expire the reflog refs
$ rm -rf .git/refs/original
$ vim .git/packed-refs                  # <-- open this file and delete anything you were warned about as unchanged above.
                                        #     notice that tag refs span two lines: delete them both.
$ git gc --aggressive --prune=now       # <-- delete all irrelevant stuff
$ du -sh .
5.4M                                    # <-- that's a lot smaller

prepare_subdir_for_gitlab_import.txt

# use git >= 1.8.5.3 !

# extract subdirectory and remove references to irrelevant history
git clone --mirror REPO
cd REPO.git
git filter-branch --subdirectory-filter PATH/TO/SUBDIR --prune-empty --tag-name-filter cat -- --all
rm -rf refs/original
rm -f packed-refs

# clean up
git reflog expire --expire=now --all
git gc --aggressive --prune=now
git reflog expire --all --expire-unreachable=0
git repack -A -d
git prune