Process unshallow commits first

Process unshallow commits first

DepthGenerator marks commits reinteresting for the ones that are
reachable from unshallow commits as it walks over the revisions. Those
unshallow commits won't necessarily be processed first. Because of this,
even if a commit is reachable from unshallow commits, if it's processed
before the uninteresting commits, it will not be processed as
reinteresting and processed as uninteresting. This causes unshallow
git-fetch to be failed.

This changes DepthGenerator to process unshallow commits first
independent to their depth. This makes uninteresting flag carry work
properly.

Change-Id: I94378271cf85fbe6302cefc19a167d8cf68e1a69
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>