difftool: support repositories with .git-files

Modern versions of "git submodule" use .git-files to setup the
submodule directory.  When run in a "git submodule"-created
repository "git difftool --dir-diff" dies with the following
error:

	$ git difftool -d HEAD~
	fatal: This operation must be run in a work tree
	diff --raw --no-abbrev -z HEAD~: command returned error: 128

core.worktree is relative to the .git directory but the logic
in find_worktree() does not account for it.

Use `git rev-parse --show-toplevel` to find the worktree so that
the dir-diff feature works inside a submodule.

Reported-by: Gábor Lipták <gabor.liptak@gmail.com>
Helped-by: Jens Lehmann <jens.lehmann@web.de>
Helped-by: John Keeping <john@keeping.me.uk>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool.perl

@@ -39,24 +39,10 @@ USAGE
 
 sub find_worktree
 {
-	my ($repo) = @_;
-
 	# Git->repository->wc_path() does not honor changes to the working
 	# tree location made by $ENV{GIT_WORK_TREE} or the 'core.worktree'
 	# config variable.
-	my $worktree;
-	my $env_worktree = $ENV{GIT_WORK_TREE};
-	my $core_worktree = Git::config('core.worktree');
-
-	if (defined($env_worktree) and (length($env_worktree) > 0)) {
-		$worktree = $env_worktree;
-	} elsif (defined($core_worktree) and (length($core_worktree) > 0)) {
-		$worktree = $core_worktree;
-	} else {
-		$worktree = $repo->wc_path();
-	}
-
-	return $worktree;
+	return Git::command_oneline('rev-parse', '--show-toplevel');
 }
 
 sub print_tool_help
@@ -418,7 +404,7 @@ sub dir_diff
 	my $rc;
 	my $error = 0;
 	my $repo = Git->repository();
-	my $workdir = find_worktree($repo);
+	my $workdir = find_worktree();
 	my ($a, $b, $tmpdir, @worktree) =
 		setup_dir_diff($repo, $workdir, $symlinks);