mingw: avoid mktemp() in mkstemp() implementation

mingw: avoid mktemp() in mkstemp() implementation

The implementation of mkstemp() for MinGW uses mktemp() and open()
without the flag O_EXCL, which is racy.  It's not a security problem
for now because all of its callers only create files within the
repository (incl. worktrees).  Replace it with a call to our more
secure internal function, git_mkstemp_mode(), to prevent possible
future issues.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>