UploadPack: Avoid walking the entire project history

UploadPack: Avoid walking the entire project history

If the client presents a common commit on a side branch, and there is
a want for a disconnected branch UploadPack was walking back on the
entire history of the disconnected branch because it never would find
the common commit.

Limit our search back along any given want to be no earlier than the
oldest common commit received via a "have" line from our client.  This
prevents us from looking at all of the project history.

Bug: 301639
Change-Id: Iffaaa2250907150d6efa1cf2f2fcf59851d5267d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>