introduce "extensions" form of core.repositoryformatversion

Normally we try to avoid bumps of the whole-repository
core.repositoryformatversion field. However, it is
unavoidable if we want to safely change certain aspects of
git in a backwards-incompatible way (e.g., modifying the set
of ref tips that we must traverse to generate a list of
unreachable, safe-to-prune objects).

If we were to bump the repository version for every such
change, then any implementation understanding version `X`
would also have to understand `X-1`, `X-2`, and so forth,
even though the incompatibilities may be in orthogonal parts
of the system, and there is otherwise no reason we cannot
implement one without the other (or more importantly, that
the user cannot choose to use one feature without the other,
weighing the tradeoff in compatibility only for that
particular feature).

This patch documents the existing repositoryformatversion
strategy and introduces a new format, "1", which lets a
repository specify that it must run with an arbitrary set of
extensions. This can be used, for example:

 - to inform git that the objects should not be pruned based
   only on the reachability of the ref tips (e.g, because it
   has "clone --shared" children)

 - that the refs are stored in a format besides the usual
   "refs" and "packed-refs" directories

Because we bump to format "1", and because format "1"
requires that a running git knows about any extensions
mentioned, we know that older versions of the code will not
do something dangerous when confronted with these new
formats.

For example, if the user chooses to use database storage for
refs, they may set the "extensions.refbackend" config to
"db". Older versions of git will not understand format "1"
and bail. Versions of git which understand "1" but do not
know about "refbackend", or which know about "refbackend"
but not about the "db" backend, will refuse to run. This is
annoying, of course, but much better than the alternative of
claiming that there are no refs in the repository, or
writing to a location that other implementations will not
read.

Note that we are only defining the rules for format 1 here.
We do not ever write format 1 ourselves; it is a tool that
is meant to be used by users and future extensions to
provide safety with older implementations.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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