rebase --apply: make reflog messages match rebase --merge

rebase --apply: make reflog messages match rebase --merge

The apply backend creates slightly different reflog messages to the
merge backend when starting or finishing a rebase and when picking
commits. These differences make it harder than it needs to be to parse
the reflog (I have a script that reads the finishing messages from
rebase and it is a pain to have to accommodate two different message
formats). While it is possible to determine the backend used for a
rebase from the reflog messages, the differences are not designed for
that purpose. c2417d3af7 (rebase: drop '-i' from the reflog for
interactive-based rebases, 2020-02-15) removed the clear distinction
between the reflog messages of the two backends without complaint.

As the merge backend is the default it is likely to be the format most
common in existing reflogs. For that reason the apply backend is changed
to format its reflog messages to match the merge backend as closely as
possible. Note that there is still a difference as when committing a
conflict resolution the apply backend will use "(pick)" rather than
"(continue)" because it is not currently possible to change the message
for a single commit.

In addition to c2417d3af7 we also changed the reflog messages in
68aa495b59 (rebase: implement --merge via the interactive machinery,
2018-12-11) and 2ac0d6273f (rebase: change the default backend from "am"
to "merge", 2020-02-15). This commit makes the same change to "git
rebase --apply" that 2ac0d6273f made to "git rebase" without any backend
specific options. As the messages are changed to use an existing format
any scripts that can parse the reflog messages of the default rebase
backend should be unaffected by this change.

There are existing tests for the messages from both backends which are
adjusted to ensure that they do not get out of sync in the future.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>