Merge branch 'kg/gc-auto-windows-workaround'

"git gc --auto" opens file descriptors for the packfiles before
spawning "git repack/prune", which would upset Windows that does
not want a process to work on a file that is open by another
process.  The issue has been worked around.

* kg/gc-auto-windows-workaround:
  gc --auto: release pack files before auto packing

builtin/gc.c

@@ -615,6 +615,7 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		return -1;
 
 	if (!repository_format_precious_objects) {
+		close_all_packs(the_repository->objects);
 		if (run_command_v_opt(repack.argv, RUN_GIT_CMD))
 			return error(FAILED_RUN, repack.argv[0]);