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.

In the longer term, difftool probably needs to learn to warn the user if
changes to working tree files would be lost by copying the temporary
files back instead of simply overwriting whatever is in the working
tree with the (potentially unmodified) temporary file.

Signed-off-by: John Keeping <john@keeping.me.uk>

---
Changes since v2:
- Fix grammar issues in commit message
- Clarify final paragraph of commit message
t7800: run --dir-diff tests with and without symlinks

Currently the difftool --dir-diff tests may or may not use symlinks
depending on the operating system on which they are run.  In one case
this has caused a test failure to be noticed only on Windows when the
test also fails on Linux when difftool is invoked with --no-symlinks.

Rewrite these tests so that they do not depend on the environment but
run explicitly with both --symlinks and --no-symlinks, protecting the
--symlinks version with a SYMLINKS prerequisite.

Signed-off-by: John Keeping <john@keeping.me.uk>