Add support for the project 'upstream' attribute  in manifests.

'repo manifest -r' adds an 'upstream' attribute to the project tag for
the manifests it generates.  We can use the attribute to get a valid
branch instead of a sha1 for these types of manifests.

BUG=chromium:249566
TEST=Manual, (pending) trybots.

Change-Id: I34daa2b59aedaa8ea08f119a252aa4397710769b
Reviewed-on: https://gerrit.chromium.org/gerrit/59575
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Ryan Cui <rcui@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>
1 file changed