Re: having revprops (eg svn:log) in start-commit

After IRC conversation between myself, brane, markphip, stsp, danielsh, and
(to a less degree) julianf, I think I have a clear path forward on this now.
 The general sentiments are:

* that 'start-commit' is today useful for making a repository "truly
  read-only" (as in, not modifiable at all) is an undocumented side-effect,
  but one that we suspect folks aren't relying /precisely/ on.

* rather, the use-cases we were collectively aware of involved ensuring
  that the repository could be simply put into a state where no new
  revisions are created, and no existing revprops are modified.

* changing 'start-commit' to be invoked after txn creation and txnprop
  population, and to receive the txn-id as a new final arg, would:

    - allow existing start-commit hooks to continue to provide the
      don't-allow-any-new-revisions behavior, and

    - allow hook authors to create/amend their start-commit hooks to
      examine txnprops and potentially offer the early detection of
      commits destined to fail that we've been talking about.

* besides, philip's freeze/unfreeze offers a far more logical way to
  get honest-to-goodness-read-only-ness.

So, based on the above, the my new plan of action is as follows:

1. Revert the addition of the 'init-commit' hook script feature.
2. Change 'start-commit' to run after txn creation and txnprop population
    (literally, like, moving a function call down two or three lines).
3. Add the txn-id argument to the end of the 'start-commit' arg list.

Bert asked the question of what would trigger the new start-commit behavior
-- upgrade to 1.8? A format bump for the repository? Personally, I lean
towards just the 1.8 upgrade. I don't feel like this only justifies a
format bump for the repository. That said, if Philip's freeze/unfreeze
brings in a format bump, I'm happy to tie this start-commit change to that
bump, too.

-- 
C. Michael Pilato <cmpilato_at_collab.net>
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