Update gitignore and add attributes to po/

.gitignore

@@ -10,3 +10,5 @@ doc/gnupg.info*
 doc/stamp-vti
 doc/version.texi
 po/gnupg2.pot
+*.o
+*.a

po/.gitattributes

@@ -0,0 +1,7 @@
+# You should add
+#[filter "cleanpo"]
+#        clean = "awk '/^\"POT-Creation-Date:/&&!s{s=1;next};!/^#: /{print}'"
+# to your config file.
+
+/??.po     filter=cleanpo
+/??_??.po  filter=cleanpo