Re: [PATCH v2 2/3] t7800: fix tests when difftool uses --no-symlinks

John Keeping [off-list ref] writes:

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 files 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.

Good.

In the longer term, difftool probably needs to learn to warn the user
instead of overwrite any changes that have been made to the working tree
file.

Questionable.

Admittedly I do not use difftool myself, and I have long assumed
that difftool users are using the tools to _view_ the changes, but
apparently some of the tools let the user muck with what is shown,
and also apparently people seem to like the fact that they can make
changes.  So I've led to believe the "update in difftool, take the
change back to working tree, either by making symbolic links or
copying them back" behaviour was a _feature_.

It is possible that this is not universally considerd as a feature,
but if that is the case, I think the right way to do this is to tell
the tools _not_ to let the user to modify contents they show in the
first place, not letting the user modify and then warning after the
fact.