Remove extra git-dir call in git-info and use git_dir variable instead

This avoids an extra `git rev-parse --git-dir` shellout when displaying git info in prompt.

Before:

```
~/.dotfiles master $ export GIT_TRACE=1
15:43:40.670779 git.c:349               trace: built-in: git 'config' '--bool' 'prompt.showinfo'    
15:43:40.682923 git.c:349               trace: built-in: git 'rev-parse' '--git-dir'
15:43:40.690074 git.c:349               trace: built-in: git 'rev-parse' '--git-dir'
~/.dotfiles master $
```

After:

```
~/.dotfiles master $ export GIT_TRACE=1
15:43:58.904774 git.c:349               trace: built-in: git 'config' '--bool' 'prompt.showinfo'    
15:43:58.915877 git.c:349               trace: built-in: git 'rev-parse' '--git-dir'
~/.dotfiles master $
```