Download latest patch when no patch is specified

When someone does "repo download -c <project> <change>"
without specifying a patch number, by default patch 1 is
downloaded. An alternative is to look for the latest patch
and download the same when no explicit patch is given.
This commit does the same by identifying the latest patch
using "git ls-remote".

Change-Id: Ia5fa7364415f53a3d9436df4643e38f3c90ded58