Avoid git fork on the common case of repo not changing

Usually repo is upgraded only once a week, if that often.  Most of
the time we invoke HasChanges on the repo project (or even on the
manifest project) the current HEAD will resolve to the same SHA-1
as the remote tracking ref, and there are therefore no changes.

Signed-off-by: Shawn O. Pearce <sop@google.com>