move duplicated ref_newer() to remote.c

move duplicated ref_newer() to remote.c

ref_newer() appears to have been copied from builtin-send-pack.c to
http-push.c via cut and paste. This patch moves the function and its
helper unmark_and_free() to remote.c. There was a slight difference
between the two implementations, one used TMP_MARK for the mark, the
other used 1. Per Jeff King, I went with TMP_MARK as more correct.

This is in preparation for being able to call it from builtin-remote.c

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>