[RFC] Removal of LogFormatter.allow_single_merge_revision

[Ian Clatworthy]

I've been chatting with Kent Gibson about the history behind
the special attribute of LogFormatter we support called
allow_single_merge_revision. It sounds like this feature was
introduced to make the *core* LogFormatters more robust in the
face of data they couldn't handle at the time. They now can,
so I'd like to remove this feature, reducing code complexity
and (slightly) improving performance.

If anyone has a custom LogFormatter that depends on this, please
let me know. I'm happy to help you improve your LogFormatter if
required. To be clear, "allow_single_merge_revision" means

"I can't handle multiple levels - just 1 - but I can handle a
single revision off the mainline."

With the new design of log, this becomes pretty meaningless because
that single flat level can be 1.2.3 to 1.2.9 say. If a LogFormatter
can handle that, and let's little reason not to be able, then
the allow_single stuff becomes obsolete semantically.

Ian C.