Fix Git completion

Git completion in new Git versions (> 1.8) are changed. After installing
Git through Homebrew content of

/usr/local/Cellar/git/1.8.1/share/git-core/contrib/completion/git-completion.zsh

was copied to

$ZSH/git/_git

file. Path to git-completion.bash script in _git file was changed to point to
the location used by Hombrew Git version.