sequencer: fix leaking string buffer in `commit_staged_changes()`

sequencer: fix leaking string buffer in `commit_staged_changes()`

We're leaking the `rev` string buffer in various call paths. Refactor
the function to have a common exit path so that we can release its
memory reliably.

This fixes a subset of tests failing with the memory sanitizer in t3404.
But as there are more failures, we cannot yet mark the whole test suite
as passing.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>