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

t/t7800-difftool.sh

@@ -434,4 +434,18 @@ test_expect_success PERL 'difftool --no-symlinks detects conflict ' '
 	)
 '
 
+test_expect_success PERL 'difftool properly honors gitlink and core.worktree' '
+	git submodule add ./. submod/ule &&
+	(
+		cd submod/ule &&
+		test_config diff.tool checktrees &&
+		test_config difftool.checktrees.cmd '\''
+			test -d "$LOCAL" && test -d "$REMOTE" && echo good
+		'\'' &&
+		echo good >expect &&
+		git difftool --tool=checktrees --dir-diff HEAD~ >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done