mergetools: add support for guiffy

Add guiffy as difftool and mergetool

guiffy is available on Windows, Linux, and MacOS

Signed-off-by: Bill Ritcher <Bill_Ritcher@guiffy.com>
Reviewed-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

mergetools/guiffy

@@ -0,0 +1,18 @@
+diff_cmd () {
+	"$merge_tool_path" "$LOCAL" "$REMOTE"
+}
+
+merge_cmd () {
+	if $base_present
+	then
+		"$merge_tool_path" -s "$LOCAL" \
+		"$REMOTE" "$BASE" "$MERGED"
+	else
+		"$merge_tool_path" -m "$LOCAL" \
+		"$REMOTE" "$MERGED"
+	fi
+}
+
+exit_code_trustable () {
+	true
+}