combo-layer: improve merge commit handling

combo-layer: improve merge commit handling

When the head of a branch is a merge commit, combo-layer did not
record that commit as last_revision because it only considers applied
patches, and the merge commit never gets applied.

This causes problems when the merge commit leads to multiple patches
and the commit id that gets recorded only reaches some of these
patches. The next run then will try to re-apply the other patches.

This special case is now detected and dealt with by bumping
last_revision to the branch commit. The behavior where the head is a
normal commit is intentionally not changed, because some users might
prefer the traditional behavior.

(From OE-Core rev: 701bcd52c208f22a9a6c48a11a35bcf1c4e413df)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>