maintner: parse Gerrit reply bodies

The contents of a Gerrit reply are stored in the body of the commit
message for a meta commit, but we currently ignore them. Parse the
message text when we walk the tree of meta commits, and return it when
we return a CL.

The API returns these in very raw fashion - what you see in a reply
in the UI is what you get in the git commit. We don't do any parsing
either. We could add helpers on the GerritMessage object to try to
return you better formatted information.

To see what the API returns for these messages, visit this URL:
https://go-review.googlesource.com/changes/42180?o=MESSAGES. The
messages attached to a specific line and file are stored a different
way and returned via a different API. It's trickier to get those so
I'll add them in a separate commit.

Change-Id: Ibd56fb828c225b57650157d08b27034ece75848a
Reviewed-on: https://go-review.googlesource.com/42452
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed