Merge branch 'da/difftool' into maint

"git difftool" learned to handle unmerged paths correctly in
dir-diff mode.

* da/difftool:
  difftool: handle unmerged files in dir-diff mode
  difftool: initialize variables for readability

git-difftool.perl

@@ -138,6 +138,7 @@ sub setup_dir_diff
 	my %submodule;
 	my %symlink;
 	my @working_tree = ();
+	my %working_tree_dups = ();
 	my @rawdiff = split('\0', $diffrtn);
 
 	my $i = 0;
@@ -188,6 +189,10 @@ EOF
 		}
 
 		if ($rmode ne $null_mode) {
+			# Avoid duplicate working_tree entries
+			if ($working_tree_dups{$dst_path}++) {
+				next;
+			}
 			my ($use, $wt_sha1) = use_wt_file($repo, $workdir,
 							  $dst_path, $rsha1);
 			if ($use) {
@@ -273,7 +278,7 @@ EOF
 	# temporary file to both the left and right directories to show the
 	# change in the recorded SHA1 for the submodule.
 	for my $path (keys %submodule) {
-		my $ok;
+		my $ok = 0;
 		if (defined($submodule{$path}{left})) {
 			$ok = write_to_file("$ldir/$path",
 				"Subproject commit $submodule{$path}{left}");
@@ -289,7 +294,7 @@ EOF
 	# shows only the link itself, not the contents of the link target.
 	# This loop replicates that behavior.
 	for my $path (keys %symlink) {
-		my $ok;
+		my $ok = 0;
 		if (defined($symlink{$path}{left})) {
 			$ok = write_to_file("$ldir/$path",
 					$symlink{$path}{left});

t/t7800-difftool.sh

@@ -419,6 +419,29 @@ run_dir_diff_test 'difftool --dir-diff when worktree file is missing' '
 	grep file2 output
 '
 
+run_dir_diff_test 'difftool --dir-diff with unmerged files' '
+	test_when_finished git reset --hard &&
+	test_config difftool.echo.cmd "echo ok" &&
+	git checkout -B conflict-a &&
+	git checkout -B conflict-b &&
+	git checkout conflict-a &&
+	echo a >>file &&
+	git add file &&
+	git commit -m conflict-a &&
+	git checkout conflict-b &&
+	echo b >>file &&
+	git add file &&
+	git commit -m conflict-b &&
+	git checkout master &&
+	git merge conflict-a &&
+	test_must_fail git merge conflict-b &&
+	cat >expect <<-EOF &&
+		ok
+	EOF
+	git difftool --dir-diff $symlinks -t echo >actual &&
+	test_cmp expect actual
+'
+
 write_script .git/CHECK_SYMLINKS <<\EOF
 for f in file file2 sub/sub
 do