t7800: fix tests when difftool uses --no-symlinks

When 'git difftool --dir-diff' is using --no-symlinks (either explicitly
or implicitly because it's running on Windows), any working tree files
that have been copied to the temporary directory are copied back after
the difftool completes.

Because an earlier test uses "git add .", the "output" file used by
tests is tracked by Git and the following sequence occurs during some
tests:

1) the shell opens "output" to redirect the difftool output
2) difftool copies the empty "output" to the temporary directory
3) difftool runs "ls" which writes to "output"
4) difftool copies the empty "output" file back over the output of the
   command
5) the output file doesn't contain the expected output, causing the
   test to fail

Instead of adding all changes, explicitly add only the files that the
test is using, allowing later tests to write their result files into the
working tree.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7800-difftool.sh

@@ -314,7 +314,7 @@ test_expect_success PERL 'setup change in subdirectory' '
 	git commit -m "added sub/sub" &&
 	echo test >>file &&
 	echo test >>sub/sub &&
-	git add . &&
+	git add file sub/sub &&
 	git commit -m "modified both"
 '