cmd.Output() will return an error when the git command ran successfully
but had no output. To handle this, we can check Stderr, as we expect it to
be populated for any ExitErrors or otherwise when there is a command
failure.
This allows for propagation of this error handling up the call chain, so
we are now returning errors if the call to git fails instead of just
handing off an empty BranchConfig and suppressing the errors.
Additionally, I've removed some more naked returns that I found in
pkg/cmd/pr/create.go createRun
e1423cdbbf