internals: document bundle2 format
It seems like a good idea to have thorough documentation of the bundle2
data format, including the format of each part and the capabilities.
The added documentation is far from complete. For example, we don't
fully capture the semantics of each capability and part. But a start
is better than nothing, which was pretty much where we were before.
Differential Revision: https://phab.mercurial-scm.org/D2298
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t