Merge branch 'jk/difftool-no-overwrite-on-copyback'

Try to be careful when difftool backend allows the user to write
into the temporary files being shown *and* the user makes changes
to the working tree at the same time. One of the changes has to be
lost in such a case, but at least tell the user what he did.

* jk/difftool-no-overwrite-on-copyback:
  t7800: run --dir-diff tests with and without symlinks
  t7800: fix tests when difftool uses --no-symlinks
  t7800: don't hide grep output
  difftool: don't overwrite modified files
  t7800: move '--symlinks' specific test to the end