sequencer: use rebase_path_message()
Rather than constructing the path in a struct strbuf use the ready
made function to get the path name instead. This was the last
remaining use of the strbuf so remove it as well.
As with the previous patch we now use a hard coded string rather than
git_dir() when constructing the path. This is safe for the same
reason (make_patch() is only called when rebasing) and is protected by
the assertion added in the previous patch.
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>