Merge branch 'da/difftool' into maint

The code to prepare the working tree side of temporary directory
for the "dir-diff" feature forgot that symbolic links need not be
copied (or symlinked) to the temporary area, as the code already
special cases and overwrites them.  Besides, it was wrong to try
computing the object name of the target of symbolic link, which may
not even exist or may be a directory.

* da/difftool:
  difftool: ignore symbolic links in use_wt_file

git-difftool.perl

@@ -70,9 +70,7 @@ sub use_wt_file
 	my ($repo, $workdir, $file, $sha1) = @_;
 	my $null_sha1 = '0' x 40;
 
-	if (! -e "$workdir/$file") {
-		# If the file doesn't exist in the working tree, we cannot
-		# use it.
+	if (-l "$workdir/$file" || ! -e _) {
 		return (0, $null_sha1);
 	}
 

t/t7800-difftool.sh

@@ -504,4 +504,23 @@ test_expect_success PERL 'difftool properly honors gitlink and core.worktree' '
 	)
 '
 
+test_expect_success PERL,SYMLINKS 'difftool --dir-diff symlinked directories' '
+	git init dirlinks &&
+	(
+		cd dirlinks &&
+		git config diff.tool checktrees &&
+		git config difftool.checktrees.cmd "echo good" &&
+		mkdir foo &&
+		: >foo/bar &&
+		git add foo/bar &&
+		test_commit symlink-one &&
+		ln -s foo link &&
+		git add link &&
+		test_commit symlink-two &&
+		echo good >expect &&
+		git difftool --tool=checktrees --dir-diff HEAD~ >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done