Rewrite the only use of "mktemp()" that is subject to TOCTOU race
and Stop using the insecure "mktemp()" function.
* rs/ban-mktemp:
compat: remove gitmkdtemp()
banned.h: ban mktemp(3)
compat: remove mingw_mktemp()
compat: use git_mkdtemp()
wrapper: add git_mkdtemp()