The existing mechanism for providing options to remote fetch/push calls,
and subsequently to transports, is unsatisfactory.  It requires an
options structure to avoid breaking the API and callback signatures.

1. Introduce `git_remote_connect_options` to satisfy those needs.

2. Add a new remote connection API, `git_remote_connect_ext` that will
   take this new options structure.  Existing `git_remote_connect` calls
   will proxy to that.  `git_remote_fetch` and `git_remote_push` will
   proxy their fetch/push options to that as well.

3. Define the interaction between `git_remote_connect` and fetch/push.
   Connect _may_ be called before fetch/push, but _need not_ be.  The
   semantics of which options would be used for these operations was
   not specified if you specify options for both connect _and_ fetch.
   Now these are defined that the fetch or push options will be used
   _if_ they were specified.  Otherwise, the connect options will be
   used if they were specified.  Otherwise, the library's defaults will
   be used.

4. Update the transports to understand `git_remote_connect_options`.
   This is a breaking change to the systems API.

Commit b1a6c316a6070fac4ab1ec5792979838f7145c39 moved auto-refresh into
the pack backend, and added a comment accordingly. Commit
43820f204ea32503b4083e3b6b83f30a0a0031c9 moved auto-refresh back *out*
of backends into the ODB layer, but didn't update the comment.

Introduce a function to create an email from a diff and multiple inputs
about the source of the diff.

Creating an email from a diff requires many more inputs, and should be
discouraged in favor of building directly from a commit, and is thus in
the `sys` namespace.

filter: filter drivers stop taking git_buf as user input