t7800: fix quoting

When passing a command-line to call an external diff command to the
difftool, we must be prepared for paths containing special characters,
e.g. backslashes in the temporary directory's path on Windows.

This patch is needed in preparation for the next commit, which will
make the MinGW version of Git *not* rewrite TMP to use forward slashes
instead of backslashes.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7800-difftool.sh

@@ -332,7 +332,7 @@ test_expect_success 'difftool --extcmd cat arg1' '
 test_expect_success 'difftool --extcmd cat arg2' '
 	echo branch >expect &&
 	git difftool --no-prompt \
-		--extcmd sh\ -c\ \"cat\ \$2\" branch >actual &&
+		--extcmd sh\ -c\ \"cat\ \\\"\$2\\\"\" branch >actual &&
 	test_cmp expect actual
 '