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>