Introduce `git_fs_path`, which operates on generic filesystem paths.
`git_path` will be kept for only git-specific path functionality (for
example, checking for `.git` in a path).

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.

This change introduces git_odb_write_multi_pack_index(), which creates a
`multi-pack-index` file from all the `.pack` files that have been loaded
in the ODB.

Fixes: #5399

This change adds the git_midx_writer_* functions to allow to
write and create `multi-pack-index` files from `.idx`/`.pack` files.

Part of: #5399

This change adds the git_midx_writer_* functions to allow to
write and create `multi-pack-index` files from `.idx`/`.pack` files.

Part of: #5399

The very last check in the freshness check for the midx was wrong ><
This was also because this function was not tested.