libgit2 has two distinct requirements that were previously solved by
`git_buf`.  We require:

1. A general purpose string class that provides a number of utility APIs
   for manipulating data (eg, concatenating, truncating, etc).
2. A structure that we can use to return strings to callers that they
   can take ownership of.

By using a single class (`git_buf`) for both of these purposes, we have
confused the API to the point that refactorings are difficult and
reasoning about correctness is also difficult.

Move the utility class `git_buf` to be called `git_str`: this represents
its general purpose, as an internal string buffer class.  The name also
is an homage to Junio Hamano ("gitstr").

The public API remains `git_buf`, and has a much smaller footprint.  It
is generally only used as an "out" param with strict requirements that
follow the documentation.  (Exceptions exist for some legacy APIs to
avoid breaking callers unnecessarily.)

Utility functions exist to convert a user-specified `git_buf` to a
`git_str` so that we can call internal functions, then converting it
back again.

Provide a utility method on a url to determine if it matches any pattern
in a comma-separated list, similar to what one would find in `NO_PROXY`
environment variables.

Provide a method to determine if a given URL matches a host:port pattern
like the ones found in `NO_PROXY` environment variables.

Using a callback to set a resolve_url is not particularly idiomatic.
Deprecate it in favor of the `set_instance_url` and
`set_instance_pushurl` functions which can now be called from the
`git_remote_ready_cb` callback.

Introduce a new callback that fires when the remote is ready to connect.

Users may want to override the URL on a particular instance of a remote,
instead of updating the configuration.  Previously, users could use a
callback to do this, but this is not particularly idiomatic.

IPv6 addresses should be used identically internally; we should not
denote them with brackets in one operating system and without them in
another.