git/diff: clean up diffs

We were overzealous about showing the changed
header, as well as setting a junk variable for
files that didn't exist; fix both.

sys/src/cmd/git/diff

@@ -26,16 +26,22 @@ fn lsdirty {
 		git/query -c $commit HEAD | subst '^..'
 }
 
+showed=()
 mntgen /mnt/scratch
 bind $branch/tree/ /mnt/scratch/a
 bind . /mnt/scratch/b
-echo diff `{git/query $commit} uncommitted
 for(f in `$nl{lsdirty | sort | uniq}){
+	if(~ $#showed 0){
+		echo diff `{git/query $commit} uncommitted
+		showed=1
+	}
 	cd /mnt/scratch
+	a=a/$f
+	b=b/$f
 	if(! test -f a/$f)
-		orig=/dev/null
+		a=/dev/null
 	if(! test -f b/$f)
-		f=/dev/null
-	diff -u a/$f b/$f
+		b=/dev/null
+	diff -u $a $b
 }
 exit ''