Windows has problems changing the mtime when the file is write protected,
even by the owner of said file.
Add a Windows-only workaround to change the mode if necessary before
trying to change the mtime.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Steffen Prohaska <prohaska@zib.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>