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

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 <redacted>
Signed-off-by: John Keeping <redacted>
Signed-off-by: Junio C Hamano <redacted>