bugfix: git HEAD not pointing to a branch

[SwiftPush]

The bug

Steps to recreate

1. Have Git's HEAD not pointing to a branch, but instead a commit
   1. For example, during a rebase or in detached head state

What Happens

An error is printed to the console and no prompt appears

>>> git checkout 92a572d
fatal: ref HEAD is not a symbolic ref
>>>

What should happen

1. No error should be printed
2. The prompt should display the git commit hash

>>> git checkout 92a572d                         (master+18)
fatal: ref HEAD is not a symbolic ref
>>>                                              (92a572d+1742)