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>

po/.gitignore

@@ -1,15 +1,31 @@
+# autopoint
+Makefile.in.in
+Makevars.template
+Rules-quot
 boldquot.sed
 en@boldquot.header
 en@quot.header
 insert-header.sin
-Makefile
-Makefile.in
-Makefile.in.in
-Makevars.template
-POTFILES
 quot.sed
-remove-potcdate.sed
 remove-potcdate.sin
-Rules-quot
+
+# configure
+Makefile.in
+Makefile
+POTFILES
+
+# intermediate files (make)
+stamp-poT
+xz.po
+xz.1po
+xz.2po
+*.new.po
+
+# make
+remove-potcdate.sed
+xz.mo
 stamp-po
+*.gmo
+
+# cached templates (make)
 xz.pot