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.

Allow users to specify additional repository extensions that they want
to support.  For example, callers can specify that they support
`preciousObjects` and then may open repositories that support
`extensions.preciousObjects`.

Similarly, callers may opt out of supporting extensions that the library
itself supports.

Add a simple accessor for workdir paths to get an absolute on-disk path
given a repository and a relative path within it.  This is useful to
avoid copy-pasta `git_buf_joinpath` and to ensure that we validate
working directory paths while honoring `core.longpaths` settings.

Introduce `git_path_validate_filesystem` which validates (absolute) on-disk
paths and `git_path_validate_workdir` to perform validations on (absolute)
working directory paths.  These functions are useful as there may be system
limitations on on-disk paths, particularly on Windows (for example,
enforcing MAX_PATH).

For working directory paths, these limitations may be per-repository, based
on the `core.longpaths` configuration setting.

Since we're using atomic primitives to read and write into the config
map cache, we need to read/write something pointer-sized.  Use an
`intptr_t` for the config map cache.

Clarify the `git_atomic` type and functions now that we have a 64 bit
version as well (`git_atomic64`).

Provide a helper function to get the initial branch for a repository,
respecting the `init.defaultBranch` configuration option, if set, and
returning the "default default" (currently `master`) otherwise.

The function `git_repository_foreach_head` is broken, as it directly
interacts with the on-disk representation of the reference database,
thus assuming that no other refdb is used for the given repository. As
this is an internal function only and all users have been replaced,
let's remove this function.

Given a Git repository, it's non-trivial to iterate over all worktrees
that are associated with it, including the "main" repository. This
commit adds a new internal function `git_repository_foreach_worktree`
that does this for us.