rebase: don't leak on "--abort"

rebase: don't leak on "--abort"

Fix a leak in the recent 6159e7add49 (rebase --abort: improve reflog
message, 2022-10-12). Before that commit we'd strbuf_release() the
reflog message we were formatting, but when that code was refactored
to use "ropts.head_msg" the strbuf_release() was omitted.

Ideally the three users of "ropts" in cmd_rebase() should use
different "ropts" variables, in practice they're completely separate,
as this and the other user in the "switch" statement will "goto
cleanup", which won't touch "ropts".

The third caller after the "switch" is then unreachable if we take
these two branches, so all of them are getting a "{ 0 }" init'd
"ropts".

So it's OK that we're leaving a stale pointer in "ropts.head_msg",
cleaning it up was our responsibility, and it won't be used again.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>