Fixes#20754.
Details: UbuntuSourcesList.add_source() had a quick check for PPAs being
already present in the source lists. The check was looking for the PPAs
URL to be present in self.repo_urls, which should contain all valid and
enabled repositories.
The enabled check in repo_urls was incorrect. It was checking the tuple's
2nd item (which means "valid") and ignoring the 3rd item (which means
"enabled").
self.files contains tuples (line_number, valid, enabled, source_line,
comment_text). Ideally it would be using named tuples instead of
indexing, to avoid bugs like that, but Python 2.4 didn't have named
tuples, so we can't do that (yet).
577d0e43ba