Merge branch 'da/difftool-git-files' into maint

"git difftool" misbehaved when the repository is bound to the
working tree with the ".git file" mechanism, where a textual file
".git" tells us where it is.

* da/difftool-git-files:
  t7800: add a difftool test for .git-files
  difftool: support repositories with .git-files