willis: use 'revision' instead of 'tracking_branch' when necessary

Some repo git trees in the manifest have their 'revision' field set to
a valid git SHA1. This value should be used as the proper tracking
branch for those trees.

Also, instead of 'manually' figuring out the list of projects, use the
Manifest object method.

TEST=Running 'willis' in the Chrome OS repo does not show trees in
     src/third_party/chromium_build, and src/third_party/mojo as not
     matching the tracking branch anymore.

Change-Id: Ida2727fcf05126471287f2e57ad0b038e0b41318
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/209968
Reviewed-by: David James <davidjames@chromium.org>
1 file changed