Corrected location of hard-coded tmp directory

In commit 1959a18, the leading '/tmp' was mistakenly dropped from the hard-coded tmp dir location. Signed-off-by: Tim Henigan <tim.henigan@gmail.com>

Author: thenigan

git-diffall

@@ -46,7 +46,7 @@ cd "$cdup" || {
 # mktemp is not available on all platforms (missing from msysgit)
 # Use a hard-coded tmp dir if it is not available
 tmp="$(mktemp -d -t tmp.XXXXXX 2>/dev/null)" || {
-tmp=/git-diffall-tmp.$$
+tmp=/tmp/git-diffall-tmp.$$
 mkdir "$tmp" || exit 1
 }