problem with modification time of packfiles

git (2.6.1) sometimes updates the modification time of a packfile, even if it has not changed at all.

On my system this triggers quite expensive an d unnecessary backup operations, which I would prefer to avoid. Is there a simple way to keep the mtime of packfiles fixed, once they are created?

Apparently the undesired mtime update is done in sha1_file.c:freshen_file() which is called (indirectly) by write_sha1_file(). However I did not understand, why this is done.

Any clarification and pointers, how mtime can be kept constant would be appreciated.

Thanks, Andreas