revisions API users: add straightforward release_revisions()

Add a release_revisions() to various users of "struct rev_list" in
those straightforward cases where we only need to add the
release_revisions() call to the end of a block, and don't need to
e.g. refactor anything to use a "goto cleanup" pattern.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fmt-merge-msg.c

@@ -699,6 +699,7 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 			shortlog(origins.items[i].string,
 				 origins.items[i].util,
 				 head, &rev, opts, out);
+		release_revisions(&rev);
 	}
 
 	strbuf_complete_line(out);