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.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

When calling `git_path_dirname_r` on a Win32 prefix, e.g. a drive
or network share prefix, we always want to return the trailing
'/'. This does not work currently when passing in a path like
'C:', where the '/' would not be appended correctly.

Fix this by appending a '/' if we try to normalize a Win32 prefix
and there is no trailing '/'.

Getting the dirname of a filesystem root should return the filesystem
root itself. E.g. the dirname of "/" is always "/". On Windows, we
emulate this behavior and as such, we should return e.g. "C:/" if
calling dirname on "C:/". But we currently fail to do so and instead
return ".", as we do not check if we actually have a Windows prefix
before stripping off the last directory component.

Fix this by calling out to `win32_prefix_length` immediately after
stripping trailing slashes, returning early if we have a prefix.