minor refactor in getPendingCommits.
Support pushing to a branch.
cleanup.
remove unused code.
Show the push dialog again if the auth is not cancelled.
Don't show push dialog if 0 commits to push.
Addressed comments.
Merge branch 'master' into status_fix

Conflicts:
	ide/app/lib/git/commands/push.dart
Merge pull request #2294 from dart-lang/push_a_branch

Push a branch