factory_diff: Add option to only show commits on factory branch

When we want to merge factory branch back to master, this can be very
handy.

BUG=chrome-os-partner:18833
TEST=Run with '-o' and check only factory branch commits are shown.

Change-Id: I7d11b3688b74e31313b45e3f0912432da2c9085c
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/48748
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>
1 file changed