How can I get git completion to show me commit names?

Question

There's something I don't understand about Git and completion in Zsh.

I am using a Windows machine with Git (v2.11. for Windows) and Cygwin with Zsh (with Babun). When I type git checkout and hit TAB twice, I get a completion prompt for branches and object names:

Pretty neat! Now, on my OS X machine with Git (2.13.0 for Mac, installed via Homebrew) and Zsh, when I type git checkout and hit TAB twice, I only get this:

Where does the functionality in the first prompt come from? In other words, what do I have to install under OS X / Zsh to make that happen?

Some things I tried:

• I've already ruled out that it's Oh-my-Zsh's git plugin doing it, because when I disable that under Windows/Cygwin, I still get the same fancy completion.
• All I have to do in Windows starting with an empty .zshrc to get this completion to work is to run autoload -U compinit && compinit
• I tried adding fpath=(/usr/local/share/zsh/site-functions/ $fpath) to my .zshrc in the hope that it'd pick up what Homebrew installed (as mentioned in the brew info git output), but it didn't change anything.
• I tried installing zsh-completions from Homebrew, adding fpath=(/usr/local/share/zsh-completions $fpath) to my .zshrc as suggested. Still no change.
• What confuses me even more is that recently, I used a MacBook from a colleague who didn't know much about Git. He had only Git installed. He got the same extended completion without having configured anything.
• My .zcompdump, if that helps: https://pastebin.com/raw/UzrZ2NqD
• My .zshrc: https://github.com/slhck/dotfiles/blob/master/.zshrc

Some more debugging output from OS X, when I type git, then Ctrl-X, then h:

tags in context :completion::complete:git::
    argument-1 options  (_arguments __git_zsh_main _git)
    use-compctl         (_default _git)
    globbed-files       (_files _default _git)
tags in context :completion::complete:git:argument-1:
    alias-commands common-commands all-commands  (__git_zsh_main _git)
    alias-commands                               (__git_zsh_cmd_alias __git_zsh_main _git)
    common-commands                              (__git_zsh_cmd_common __git_zsh_main _git)
    all-commands                                 (__git_zsh_cmd_all __git_zsh_main _git)

Whereas when I do the same on Windows/Cygwin, I get:

tags in context :completion::complete:git::
    argument-1 options  (_arguments _git)
tags in context :completion::complete:git:argument-1:
    aliases main-porcelain-commands user-commands third-party-commands ancillary-manipulator-commands ancillary-interrogator-commands interaction-commands plumbing-manipulator-commands plumbing-interrogator-commands plumbing-sync-commands plumbing-sync-helper-commands plumbing-internal-helper-commands  (_git_commands _git)

Output of ls -l ${^fpath}/_git

ls: /Users/werner/.oh-my-zsh/completions/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/functions/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/brew/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/docker-compose/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/docker/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/gem/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/git-extras/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/git-flow/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/git/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/github/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/osx/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/python/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/rails/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/ruby/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/svn/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/vagrant/_git: No such file or directory
ls: /Users/werner/.oh-my-zsh/plugins/virtualenv/_git: No such file or directory
ls: /usr/local/share/zsh-completions/_git: No such file or directory
-rw-r--r--  1 werner  admin  336270 Dec 21 19:42 /usr/local/Cellar/zsh/5.3.1/share/zsh/functions/_git
lrwxr-xr-x  1 werner  admin      56 May 18 17:26 /usr/local/share/zsh/site-functions/_git -> ../../../Cellar/git/2.13.0/share/zsh/site-functions/_git
lrwxr-xr-x  1 werner  admin      56 May 18 17:26 /usr/local/share/zsh/site-functions/_git -> ../../../Cellar/git/2.13.0/share/zsh/site-functions/_git
lrwxr-xr-x  1 werner  admin      56 May 18 17:26 /usr/local/share/zsh/site-functions/_git -> ../../../Cellar/git/2.13.0/share/zsh/site-functions/_git

Answer 1

Not sure what exactly was wrong, but here's what I did to solve the problem:

First, make sure to open Terminal and launch Bash (/bin/bash), then:

brew uninstall zsh
rm -rf /usr/local/share/zsh/site-functions
brew install zsh

---

Apparently, though, reinstalling Git will cause the same problem to occur again:

brew reinstall git

Once I do this, the completion is back to its reduced form. If that is the case, you can remove the Homebrew-installed Git completions:

rm /usr/local/share/zsh/site-functions/git-completion.bash
rm -rf /usr/local/share/zsh/site-functions/_git

This will make the extended completions show up again.