update po/.gitignore

update po/.gitignore

Since the *.gmo files are deleted by the maintainer-clean target,
I assume they are not meant to be tracked.

Also add the other files listed in the Makefile’s clean targets
(stamp-poT, xz.po, xz.[12].po, *.new.po, xz.mo) to make sure they
are not accidentally tracked.  Most of these are intermediate
files that would not appear unless a build is interrupted or
fails.

Split the list of untracked files by origin to make it easier to
tell if files are missing in the future.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>