difftool --dir-diff: allow changing any clean working tree file

The temporary directory prepared by "difftool --dir-diff" to
show the result of a change can be modified by the user via
the tree diff program, and we try hard not to lose changes
to them after tree diff program returns to us.

However, the set of files to be copied back is computed
differently between --symlinks and --no-symlinks modes.  The
former checks all paths that start out as identical to the
working tree file, while the latter checks paths that
already had a local modification in the working tree,
allowing changes made in the tree diff program to paths that
did not have any local change to be lost.

Signed-off-by: Kenichi Saita <nitoyon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool.perl

@@ -85,13 +85,9 @@ sub exit_cleanup
 
 sub use_wt_file
 {
-	my ($repo, $workdir, $file, $sha1, $symlinks) = @_;
+	my ($repo, $workdir, $file, $sha1) = @_;
 	my $null_sha1 = '0' x 40;
 
-	if ($sha1 ne $null_sha1 and not $symlinks) {
-		return 0;
-	}
-
 	if (! -e "$workdir/$file") {
 		# If the file doesn't exist in the working tree, we cannot
 		# use it.
@@ -213,8 +209,7 @@ EOF
 
 		if ($rmode ne $null_mode) {
 			my ($use, $wt_sha1) = use_wt_file($repo, $workdir,
-							  $dst_path, $rsha1,
-							  $symlinks);
+							  $dst_path, $rsha1);
 			if ($use) {
 				push @working_tree, $dst_path;
 				$wtindex .= "$rmode $wt_sha1\t$dst_path\0";

t/t7800-difftool.sh

@@ -385,6 +385,25 @@ test_expect_success PERL,SYMLINKS 'difftool --dir-diff --symlink without unstage
 	test_cmp actual expect
 '
 
+write_script modify-right-file <<\EOF
+echo "new content" >"$2/file"
+EOF
+
+run_dir_diff_test 'difftool --dir-diff syncs worktree with unstaged change' '
+	test_when_finished git reset --hard &&
+	echo "orig content" >file &&
+	git difftool -d $symlinks --extcmd "$(pwd)/modify-right-file" branch &&
+	echo "new content" >expect &&
+	test_cmp expect file
+'
+
+run_dir_diff_test 'difftool --dir-diff syncs worktree without unstaged change' '
+	test_when_finished git reset --hard &&
+	git difftool -d $symlinks --extcmd "$(pwd)/modify-right-file" branch &&
+	echo "new content" >expect &&
+	test_cmp expect file
+'
+
 write_script modify-file <<\EOF
 echo "new content" >file
 EOF