Hook up the -o flag from Hannes Achleitner (changeset 25364cc).

This adds handling for GIT_DIFF_IMAGE_OUTPUT_DIR as an environment variable,
and creates a filename for each comparison to pass through the -o flag.

git_diff_image

@@ -63,5 +63,19 @@ then
   e_flag='-e'
 fi
 
+o_flag=''
+if [ -n "${GIT_DIFF_IMAGE_OUTPUT_DIR-}" ]
+then
+    mkdir -p "$GIT_DIFF_IMAGE_OUTPUT_DIR"
+    destfile=''
+    if [ "$name1" -ne '/dev/null' ]
+    then
+        destfile=$(basename "$name1")
+    else
+        destfile=$(basename "$name2")
+    fi
+    o_flag="-o $GIT_DIFF_IMAGE_OUTPUT_DIR/$destfile"
+fi
 
-exec "$thisdir/diff-image" $e_flag -n "$name1" -N "$name2" "$f1" "$f2"
+
+exec "$thisdir/diff-image" $e_flag $o_flag -n "$name1" -N "$name2" "$f1" "$f2"