More permissive "git-rm --cached" behavior without -f.

More permissive "git-rm --cached" behavior without -f.

In the previous behavior, "git-rm --cached" (without -f) had the same
restriction as "git-rm". This forced the user to use the -f flag in
situations which weren't actually dangerous, like:

$ git add foo           # oops, I didn't want this
$ git rm --cached foo   # back to initial situation

Previously, the index had to match the file *and* the HEAD. With
--cached, the index must now match the file *or* the HEAD. The behavior
without --cached is unchanged, but provides better error messages.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>