blame: allow to blame paths freshly added to the index

blame: allow to blame paths freshly added to the index

When blaming files, changes in the work tree are taken into account
and displayed as being "Not Committed Yet".

However, when blaming a file that is not known to the current HEAD,
git blame fails with `no such path 'foo' in HEAD`, even when the file
was git add'ed.

Allowing such a blame is useful when the new file added to the index
(not yet committed) was created by renaming an existing file.  It
also is useful when the new file was created from pieces already in
HEAD, moved or copied from other files and blaming with copy
detection (i.e. "-C").

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>