git-rm: don't remove newly added file without -f

git-rm: don't remove newly added file without -f

Given this set of commands:

  $ echo "newly added file" >new
  $ git add new
  $ git rm new

the file "new" was previously removed from the working
directory and the index. Because it was not in HEAD, it is
available only by searching for unreachable objects.

Instead, we now err on the safe side and refuse to remove
a file which is not referenced by HEAD.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>