sequencer: fast-forward `merge` commands, if possible
Just like with regular `pick` commands, if we are trying to rebase a
merge commit, we now test whether the parents of said commit match HEAD
and the commits to be merged, and fast-forward if possible.
This is not only faster, but also avoids unnecessary proliferation of
new objects.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>