git: Use FindCheckouts() when determining if a project supports content merging

A project may map to multiple checkouts. When checking if a project
supports content merging, we only care for the version that is actually
patchable.

Change the handling to work on the patchable checkout rather than
the projects dictionary. Get rid of the simple projects dictionary
since project -> attributes is not a 1:1 mapping any more.

BUG=chromium:330225
TEST=buildbot/run_tests
TEST=trybot on pre-cq-group, lumpy-{paladin,canary}, paladin-master,
     link-tot-paladin

Change-Id: I1a29854deb700d99c7cbfbe295b60e539331ce16
Reviewed-on: https://chromium-review.googlesource.com/185604
Tested-by: Gaurav Shah <gauravsh@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Gaurav Shah <gauravsh@chromium.org>
3 files changed