libgit2 can be built with optional, experimental sha256 support. This
allows consumers to begin testing and providing feedback for our sha256
support while we continue to develop it, and allows us to make API
breaking changes while we iterate on a final sha256 implementation.

The results will be `git2-experimental.dll` and installed as
`git2-experimental.h` to avoid confusion with a production libgit2.

Linux has a /usr/include/features.h, which gets confusing; update this
to `git2_features.h` and move it into the `util` directory.

Teach the loose object database how to cope with SHA256 objects.

Tidy up `nfmt` / `pathfmt`.

Move the arguments to `git_odb_loose` into an options structure.

Allow the object database to take an oid type that it supports.  This
oid type will be used to validate the objects that the backends provide.

Users will need to be able to specify the object id type for the given
object database; add a new `git_odb_options` with that option.

The git_odb_hash helper functions should not assume SHA1, and instead
should be given the oid type that they're producing.

We intentionally separate oid types from hash types; a hash is a generic
hunk of bytes, an object id has meaning and backs an object on disk.  As
a result of this separation, we need a 1:1 mapping.

libgit2's current default oid type is SHA1, set a public macro for that.

`git_oid`s now have a type, and we require the oid type when creating
the object id from creation functions.

Provide helper functions to provide information about the object id size
given its type.

We require an enumeration to help us identify the different types of
object IDs.  Currently, we only support SHA1 but we will support SHA256
in the future.

Callers should not assume the layout of the oid structure; provide them
a macro that defines the null / zero sha1 object id.

In preparation for SHA256 support, `GIT_OID_RAWSZ` and `GIT_OID_HEXSZ`
need to indicate that they're the size of _SHA1_ OIDs.

repo: allow administrator to own the configuration

status: update documentation for default options

config: update config entry iteration lifecycle

Update our ownership checks that were introduced in libgit2 v1.4.3
(to combat CVE 2022-24765). These were not compatible with git's; git
itself allows administrators to own the path. Our checks now match
this behavior.

Look for `getentropy` and flag its existence.

transport: introduce `git_transport_smart_remote_connect_options`

Snuck in during refactoring; let's get rid of it.

A couple of simplications around mwindow

Add a .clang-format with our style

Co-authored-by: Edward Thomson <ethomson@github.com>

CLI: progress updates

refs: fix missing error message

docs: fix couple of typos

revparse: Remove error-prone, redundant test

midx: do not verify the checksum on load