use GIT_DIR instead of /var/tmp

Not every system (will not one microsoft windows system) have /var/tmp,
whereas using GIT_DIR for random temporary files is more or less established.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-reset.sh

@@ -3,7 +3,7 @@
 USAGE='[--mixed | --soft | --hard]  [<commit-ish>]'
 . git-sh-setup
 
-tmp=/var/tmp/reset.$$
+tmp=${GIT_DIR}/reset.$$
 trap 'rm -f $tmp-*' 0 1 2 3 15
 
 reset_type=--mixed