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

Commit 02c5631 (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: Kevin Bracey <kevin@bracey.fi>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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