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.

The static test data is erroneously initialized with a length of 0 for
three of the strings.  This means the tests are not actually examining
those strings.  Provide the length.

It was reported that, given a file "abc.txt", a diff will be shown if an
empty directory "abb/" is created, but not if "abd/" is created. Add a
test to verify that we do the right thing here and do not depend on any
ordering.

`cvar` is an unhelpful name.  Refactor its usage to `configmap` for more
clarity.

Move to the `git_error` name in the internal API for error-related
functions.

Use the new object_type enumeration names within the codebase.

Use the new-style index names throughout our own codebase.

Reported by Coverity, CID 1393678-1393697.