gitk uses a predictable ".gitk-tmp.$PID" pattern when generating
a temporary directory.

Use "mktemp -d .gitk-tmp.XXXXXX" to harden gitk against someone
seeding /tmp with files matching the pid pattern.
Signed-off-by: NDavid Aguilar <davvid@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>