fast-import: disallow "." and ".." path components

fast-import: disallow "." and ".." path components

If a user specified e.g.
   M 100644 :1 ../some-file
then fast-import previously would happily create a git history where
there is a tree in the top-level directory named "..", and with a file
inside that directory named "some-file".  The top-level ".." directory
causes problems.  While git checkout will die with errors and fsck will
report hasDotdot problems, the user is going to have problems trying to
remove the problematic file.  Simply avoid creating this bad history in
the first place.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>