Merge branch 'dk/gc-idx-wo-pack' into maint

Having a leftover .idx file without corresponding .pack file in
the repository hurts performance; "git gc" learned to prune them.

We may want to do the same for .bitmap (and notice but not prune
.keep) without corresponding .pack, but that can be a separate
topic.

* dk/gc-idx-wo-pack:
  gc: remove garbage .idx files from pack dir
  t5304: test cleaning pack garbage
  prepare_packed_git(): refactor garbage reporting in pack directory

path.c

@@ -147,7 +147,7 @@ void report_linked_checkout_garbage(void)
 		strbuf_setlen(&sb, len);
 		strbuf_addstr(&sb, path);
 		if (file_exists(sb.buf))
-			report_garbage("unused in linked checkout", sb.buf);
+			report_garbage(PACKDIR_FILE_GARBAGE, sb.buf);
 	}
 	strbuf_release(&sb);
 }