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>

builtin/fast-export.c

@@ -1275,6 +1275,7 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 		printf("done\n");
 
 	refspec_clear(&refspecs);
+	release_revisions(&revs);
 
 	return 0;
 }