Join multiline comments by ' '. Each tag starts new comment.

With multiple single line comments, if they both have tags
we want to remove them correctly if we have strip_comment_tags=True.

I've joined multiline comments by ' ' instead of '\n'.  This means
we can't add a \n in a comment if we wanted to, but the general
case when adding a multi-line comment in a file is that we do want
to treat it as a single comment. This will make it easier to merge
po_entries from two different files.

Test Plan:
tools/runtests.py tools/runtests.py third_party/babel/messages/tests/extract.py exercises.babel_test.ExtractStringsTest kake.compile_all_pot_test

Auditors: csilvers