We introduced post submit diffs that are posted when a change is
submitted without review (only with review of a previous patch-set, that
got a sticky vote).
I93a6374b created limits for messages, but mentioned in the javadoc that
autogenerated change messages are not enforced.
We do the following:
1. Ensure the limit for change messages that are auto-generated is not
regarded when adding an unrelated comment (filter those change messages
when computing the cumulative messages size).
2. Ensure huge change messages for post submit diff are not allowed, and
shorten them in this case.
Change-Id: If195140f916a1af6bde123928cc61ef48320a174
081527c0a7