Autocompletion does not work for git aliases

I've never been able to get autocompletion working for git aliases I've created. My shell-fu is very weak, but I've spent several days debugging this over the course of months to no avail. Hopefully I'm just doing something stupid.

For comparison, here's what git branch <tab> looks like:

➜  .dotfiles git:(master) ✗ git branch
HEAD                              master                            origin/git-post-push-compare-url
ORIG_HEAD                         origin/HEAD                       origin/master

and here's gb <tab> (my git branch alias):

➜  .dotfiles git:(master) ✗ gb
LICENSE.md     atom.symlink/  clojure/       git/           js/            ruby/          system/        zsh/
README.md      bin/           functions/     homebrew/      osx/           script/        vim/

I added a couple debug messages to the completion script, and it looks like the test for file existence is failing. The following prints "Here," but never "There":

# completion.zsh
completion='$(brew --prefix)/share/zsh/site-functions/_git'

echo "Here"
if test -f $completion
then
  echo "There"
  source $completion
fi

If I cd into that directory and ls -l, I see the following:

➜  site-functions git:(master) ls -l
total 28
lrwxr-xr-x 1 kshipley admin 61 May  7 14:15 _docker -> ../../../Cellar/docker/1.6.0/share/zsh/site-functions/_docker
lrwxr-xr-x 1 kshipley admin 58 Aug 17 23:10 _flow -> ../../../Cellar/flow/0.14.0/share/zsh/site-functions/_flow
lrwxr-xr-x 1 kshipley admin 55 Sep  6 08:58 _git -> ../../../Cellar/git/2.5.1/share/zsh/site-functions/_git
lrwxr-xr-x 1 kshipley admin 60 Mar 25 11:42 _hg -> ../../../Cellar/mercurial/3.3.2/share/zsh/site-functions/_hg
lrwxr-xr-x 1 kshipley admin 55 Sep  6 09:50 _hub -> ../../../Cellar/hub/2.2.1/share/zsh/site-functions/_hub*
lrwxr-xr-x 1 kshipley admin 62 Jan 11  2015 _lein -> ../../../Cellar/leiningen/2.5.0/share/zsh/site-functions/_lein
lrwxr-xr-x 1 kshipley admin 70 Sep  6 08:58 git-completion.bash -> ../../../Cellar/git/2.5.1/share/zsh/site-functions/git-completion.bas

(And the files do exist on the other side of the symlink.)

I reinstalled git and hub this morning, figured out that ZSH was pointing to the Mac-default instead of my Homebrew-installed version, and tried every little bit of cleanup I could think of, but still no luck.

Here's my fork of the repo. I've made a few small tweaks (mostly removing things), so it's possible I removed something important.

Any ideas?

Start with completion.zsh and reproduce the difference between git branch <tab> and the gb alias. Inspect the _git completion path under Homebrew's share/zsh/site-functions and compare it with the fork's changes. Done means git aliases receive the expected git completion behavior in ZSH.