Branch name tab expansion does not work with aliases

I have the following alias in my `.gitconfig` (To test, I cleared out everything else so my gitconfig only contained the following).

``` ini
[alias]
    co = checkout
```

A long time ago, Posh-Git tab expansion for branch names would work with this. But now it doesn't.

Expected: Given a repo with branches, `git co x{tab}` would expand a branch name that starts with "x"
Actual: Nothing happens.
