test_git: Do not include milliseconds into dates because git uses just seconds.
test_git: Add a comment before datetime import.
git: Add a new method to Git: get_commit_from_hash().
git: Refactor Git.get_commit_from_hash().
git: Add an explicit line end to a regexp in Git._get_branches_from_ls_remote_output().
test_git: Refactor GitGetCommitFromHashTests.
git: Add a new method: Git.get_commit_for_branch().
git: Refactor Git.get_commit_from_hash() and Git.get_commit_for_branch().