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 has been caught by running the test suite with an MSVC-built Git:
in contrast to the MINGW one, it does not rewrite `$TMP` to use forward
slashes instead of backslashes.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

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
 '