Stop searching for branch/topic split at "refs/"

We might have "refs/${username}/*" acting as a sandbox area,
and a single-digit user name would have "refs/a/a/" shorter than
"refs/heads/" making the split logic abort too early before finding
a match.  Only abort the split once we've reached "refs/" as the
only remaining branch part.

Change-Id: I3fe0c54a2d7772229beb53ba3991e7d25bc1c8a9
Signed-off-by: Shawn O. Pearce <sop@google.com>