[RFC/PATCH 0/2] bumping repository format version

[Jeff King <peff@peff.net>]

We've managed to avoid bumping core.repositoryformatversion for the past
10 years, which is great. But I think there are some looming features
that are going to need it. The most obvious one is changing the ref
storage, where we need some way to tell older gits "no, please don't
think that taking 'refs/heads/foo.lock' is sufficient to actually lock".

The first patch in this series is an attempt to pave the way for version
bumps like this in as painless a way as possible, by letting us mark
incompatible "extensions" by name. That way we can version things like
"how do you lock a ref" independent of the main repositoryformatversion
setting (just like we do for index version, for example).

See the explanation in the first patch for more details. The second
patch shows another use of the "extension" feature to provide safety
in shared-object repos against older versions of git.

  [1/2]: introduce "extensions" form of core.repositoryformatversion
  [2/2]: introduce "preciousObjects" repository extension

-Peff