Teach `git_path_is_valid` to respect `core.longpaths`.  Add helper
methods to validate length and set the error message appropriately.

Add a `git_str` based validity check; the existing `git_path_is_valid`
defers to it.

Provide a mechanism for users to limit the number of characters that are
examined; `git_fs_path_str_is_valid` and friends will only examine up to
`str->size` bytes.

`git_fs_path_is_valid` delegates to these new functions by passing
`SIZE_MAX` (instead of doing a `strlen`), which is a sentinel value
meaning "look for a NUL terminator".

Since we're returning a boolean about validation, the name is more
properly "is valid".

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.

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.

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.