Following cases need to the handled:
1. If two patchsets are only different on commit message, their coverage data should be shared.
2. If two patchsets are only different due to trivial rebasing, their coverage data should be shared as well, though needs rebasing.
3. A special case of 2, whenever a CL lands, a new patchset is automatically created due to rebasing, and it is possible that a file in the CL be changed by another commit landed after the CQ build started and before the CL landed.