difftool: fix dir-diff when file does not exist in working tree

Commit 02c56314 (difftool --dir-diff: symlink all files matching the
working tree, 2013-03-14) does not handle the case where a file that is
being compared does not exist in the working tree.  Fix this by checking
for existence explicitly before running git-hash-object.
Reported-by: NKevin Bracey <kevin@bracey.fi>
Signed-off-by: NJohn Keeping <john@keeping.me.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-difftool.perl

@@ -92,6 +92,12 @@ sub use_wt_file
 		return 0;
 	}
 
+	if (! -e "$workdir/$file") {
+		# If the file doesn't exist in the working tree, we cannot
+		# use it.
+		return (0, $null_sha1);
+	}
+
 	my $wt_sha1 = $repo->command_oneline('hash-object', "$workdir/$file");
 	my $use = ($sha1 eq $null_sha1) || ($sha1 eq $wt_sha1);
 	return ($use, $wt_sha1);

t/t7800-difftool.sh

@@ -356,6 +356,13 @@ run_dir_diff_test 'difftool --dir-diff from subdirectory' '
 	)
 '
 
+run_dir_diff_test 'difftool --dir-diff when worktree file is missing' '
+	test_when_finished git reset --hard &&
+	rm file2 &&
+	git difftool --dir-diff $symlinks --extcmd ls branch master >output &&
+	grep file2 output
+'
+
 write_script .git/CHECK_SYMLINKS <<\EOF
 for f in file file2 sub/sub
 do