[autotest] add a quickmerge sentinel file

Add a quickmerge sentinel file to autotest. It's presence in the sysroot
copy of autotest will indicate that a quickmerge has been performed
(more recently than the most recent emerge of autotest).

CQ-DEPEND=CL:I75d4cd058282c3ecee279849da53cdcafade2cf1
BUG=chromium:248545
TEST=See other CL

Change-Id: Ib6269894d829b8d4837d5aa15a3de8202c887973
Reviewed-on: https://gerrit.chromium.org/gerrit/58901
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
1 file changed