Windows: fix utime() for read-only files

Windows: fix utime() for read-only files

Starting with 5256b00 (Use git_mkstemp_mode instead of plain mkstemp to
create object files, 2010-02-22) utime() is invoked on read-only files.
This is not allowed on Windows and results in many warnings of the form

failed utime() on .git/objects/23/tmp_obj_VlgHlc: Permission denied

during a repack.  Fix it by making the file temporarily writable.

Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>