`git_filter_list_stream_data` takes user input in a `git_buf`.
`git_buf` should only be used when libgit2 itself needs to allocate data
and returned to a user that they can free when they wish.  Replace it
with `git_filter_list_stream_buffer` that takes a data buffer and
length.

test: clean up memory leaks

Support `core.longpaths` on Windows

ci: use GitHub for storing mingw-w64 build dependency

Working directory path validation

Ensure that a repository's path (at initialization or open time) is
valid.  On Windows systems, this means that the longest known path
beneath the repository will fit within MAX_PATH: this is a lock file for
a loose object within the repository itself.

Other paths, like a very long loose reference, may fail to be opened
after the repository is opened.  These variable length paths will be
checked when they are accessed themselves.  This new functionality is
done at open to prevent needlessly checking every file in the gitdir
(eg, `MERGE_HEAD`) for its length when we could instead check once at
repository open time.

On Windows, we need to enforce MAX_PATH for loose references and their
reflogs.  Ensure that any path - including the lock file - would fit
within the 260 character maximum.

We do not honor core.longpaths for loose reference files or reflogs.
core.longpaths only applies to paths in the working directory.

Let `git_path_find_dir` simply take a `git_buf` that contains a
directory or a file, instead of trying to both join a path AND then deal
with prettifying it or its basename.  This allows consumers to join
paths themselves (and apply any necessary rules - like fitting within
MAX_PATH).

Worktree paths need to fix within MAX_PATH always, regardless of
`core.longpaths` setting.

Validate that working directory paths honor `core.longpaths` where
appropriate.  Paths to the submodule gitdirs must always honor the
operating system length restrictions; `core.longpaths` does not affect
gitdir paths.

Supply the repository for the filesystem and workdir iterators - for
workdir iterators, this is non-null and we can lookup the core.longpaths
configuration option.  (For regular filesystem iterators, this is NULL,
so core.longpaths does not apply.)

We should allow attribute files - inside working directories - to have
names longer than MAX_PATH when core.longpaths is set.
`git_attr_path__init` takes a repository to validate the path with.

The new git_repository_workdir_path function does error checking on
working directory inputs on Windows; use it to construct paths within
working directories.

Use `git_repository_workdir_path` to generate workdir paths since it
will validate the length.

Ensure that we are validating working directory paths before we try to
write to them.

We're not necessarily checking out into the working directory.  We could
be checking out into an arbitrary location.  Ensure that when we are
writing conflict data that we do it in the checkout target.

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.

There was no test ensuring that we validate `.git` paths.  We do, but
let's add a test to make sure that we never regress this.

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.

src: fix typos in header files

If we want to validate more and different types of paths, the name
`git_path_validate` makes that easier and more expressive.  We can add,
for example, `git_path_validate_foo` while the current name makes that
less ergonomic.

Introduce a function to determine the number of Unicode characters in a
given UTF-8 string.

Move the utf8 functions into a proper namespace `git_utf8` instead of
being in the namespaceless `git__` function group.  Update them to
have out-params first and use `char *` instead of `uint8_t *` to match
our API treating strings as `char *` (even if they truly contain `uchar`s
inside).

repo: remove an inappropriate use of PASSTHROUGH

repo: specify init.defaultbranch is meant to be a branch name

tests: fix variable name in list.c

Fix diff_entrycount -> diff_num_deltas doc typo

This just fixes a typo in the documentation, actual rename change was
done in 5f69a31f