downgrade the version of quickmerged package in autotest_quickmerge
downgrade the version of quickmerged package in autotest_quickmerge

After a package has been quickmerged, downgrade its version number to
'-0' by moving its database directory. This allows a subsequent run of
`build_packages` to know that the package needs to be un-merged and then
re-emerged, thus cleaning up after quickmerge.

BUG=chromium:229234
TEST=`touch ~/trunk/src/third_party/autotest/files/client/ardvark.py`
`autotest-quickmerge --board=lumpy`
`equery-lumpy belongs ardvark.py` # owned by autotest-tests-0
`build_packages`                  # autotest-tests re-emerged
`equery-lumpy belongs ardvark.py` # no matches

Change-Id: If810701a7e660dbda4c3a6b4570a8bce602d86a8
Reviewed-on: https://gerrit.chromium.org/gerrit/48404
Tested-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Commit-Queue: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite