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.

Separate the concerns of the hash functions from the git_oid functions.
The git_oid structure will need to understand either SHA1 or SHA256; the
hash functions should only deal with the appropriate one of these.

`git_strarray` is a public-facing type.  Change
`git_buf_text_common_prefix` to not use it, and just take an array of
strings instead.

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.

Introduce a utility function that compares a NUL terminated string to a
possibly not-NUL terminated string with length.  This is similar to
`strncmp` but with an added check to ensure that the lengths match (not
just the `size` portion of the two strings).

Also, add `git_commit_graph_writer_options_init`!

GCC C11 warnings

Instead of buf->"typeofbuffer"ReparseBuffer the members will be
referenced with buf->ReparseBuffer."typeofbuffer"

https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_reparse_data_buffer?redirectedfrom=MSDN
calls the union DUMMYUNIONNAME but that looks a bit cluttered.