Starting at release v1.7.9, if you ask to merge a signed tag, "git merge"
always creates a merge commit, even when the tag points at a commit that
happens to be a descendant of your current commit.
Unfortunately, this interacts rather badly for people who use --ff-only to
make sure that their branch is free of local developments. It used to be
possible to say:
$ git checkout -b frotz v1.7.9~30
$ git merge --ff-only v1.7.9
and expect that the resulting tip of frotz branch matches v1.7.9^0 (aka
the commit tagged as v1.7.9), but this fails with the updated Git with:
fatal: Not possible to fast-forward, aborting.
because a merge that merges v1.7.9 tag to v1.7.9~30 cannot be created by
fast forwarding.
We could teach users that now they have to do
$ git merge --ff-only v1.7.9^0
but it is far more pleasant for users if we DWIMmed this ourselves.
When an integrator pulls in a topic from a lieutenant via a signed tag,
even when the work done by the lieutenant happens to fast-forward, the
integrator wants to have a merge record, so the integrator will not be
asking for --ff-only when running "git pull" in such a case. Therefore,
this change should not regress the support for the use case v1.7.9 wanted
to add.
Signed-off-by: Junio C Hamano <gitster@pobox.com>
b5c9f1c1b0