Require branches to always start from commits

When creating a new branch, require that the branch points to a
commit object, and not to another type such as tag, tree, or blob.

Bug: issue 575
Change-Id: Ia0ebd25eb8d67fc113dee5a74fd51454f4bf180f
Signed-off-by: Shawn O. Pearce <sop@google.com>
2 files changed