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.

Always pass a working-directory relative path to attribute lookups
during checkout.

Checkout dry-run

Filters use a short-lived structure to keep state during an operation to
allow for caching and avoid unnecessary reallocations.  This was
previously called the "filter options", despite the fact that they
contain no configurable options.  Rename them to a "filter session" in
keeping with an "attribute session", which more accurately describes
their use (and allows us to create "filter options" in the future).

buf: remove unnecessary buf_text namespace

The `git_buf_text` namespace is unnecessary and strange.  Remove it,
just keep the functions prefixed with `git_buf`.