[RFC] deprecation of Tree.path_content_summary()

[Ian Clatworthy]

The path_content_summary API was introduced some time back to speed up
commit but it's proven to be dangerous in formats (1.14 or later) that
support content filtering. This has led to some nasty bugs such as
https://bugs.launchpad.net/bzr/+bug/405251.

FWIW, we now have a faster commit path that doesn't use this API thanks
to work by Robert. I think we ought to deprecate and/or eliminate
path_content_summary() for 2.0.

Any thoughts/objections?

Ian C.