sequencer: rework reflog message handling

sequencer: rework reflog message handling

It has been reported that "git rebase --rebase-merges" can create
corrupted reflog entries like

    e9c962f2ea0 HEAD@{8}: <binary>�: Merged in <branch> (pull request #4441)

This is due to a use-after-free bug that happens because
reflog_message() uses a static `struct strbuf` and is not called to
update the current reflog message stored in `ctx->reflog_message` when
creating the merge. This means `ctx->reflog_message` points to a stale
reflog message that has been freed by subsequent call to
reflog_message() by a command such as `reset` that used the return value
directly rather than storing the result in `ctx->reflog_message`.

Fix this by creating the reflog message nearer to where the commit is
created and storing it in a local variable which is passed as an
additional parameter to run_git_commit() rather than storing the message
in `struct replay_ctx`. This makes it harder to forget to call
`reflog_message()` before creating a commit and using a variable with a
narrower scope means that a stale value cannot carried across a from one
iteration of the loop to the next which should prevent any similar
use-after-free bugs in the future.

A existing test is modified to demonstrate that merges are now created
with the correct reflog message.

Reported-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>