mergetools/kdiff3: do not use --auto when diffing

The `kdiff3 --auto` help message is, "No GUI if all conflicts are auto-
solvable."  This flag was carried over from the original mergetool
commands.  diff_cmd() is for two-way comparisons only so remove the
superfluous flag.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/kdiff3

@@ -1,5 +1,5 @@
 diff_cmd () {
-	"$merge_tool_path" --auto \
+	"$merge_tool_path" \
 		--L1 "$MERGED (A)" --L2 "$MERGED (B)" \
 		"$LOCAL" "$REMOTE" >/dev/null 2>&1
 }