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 `repo` argument is now unnecessary.  Remove it.

Allow more advanced attribute queries using a `git_attr_options`, and
extended functions to use it.  Presently there is no additional
configuration in a `git_attr_options` beyond the flags, but this is for
future growth.

We may want to extend the attribute source; use a structure instead of
an enum.

The enum `git_attr_file_source` is better suffixed with a `_t` since
it's a type-of source.  Similarly, its members should have a matching
name.

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.

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.

Right now, we are unconditionally applying all macros found in a
gitatttributes file. But quoting gitattributes(5):

    Custom macro attributes can be defined only in top-level
    gitattributes files ($GIT_DIR/info/attributes, the .gitattributes
    file at the top level of the working tree, or the global or
    system-wide gitattributes files), not in .gitattributes files in
    working tree subdirectories. The built-in macro attribute "binary"
    is equivalent to:

So gitattribute files in subdirectories of the working tree may
explicitly _not_ contain macro definitions, but we do not currently
enforce this limitation.

This patch introduces a new parameter to the gitattributes parser that
tells whether macros are allowed in the current file or not. If set to
`false`, we will still parse macros, but silently ignore them instead of
adding them to the list of defined macros. Update all callers to
correctly determine whether the to-be-parsed file may contain macros or
not. Most importantly, when walking up the directory hierarchy, we will
only set it to `true` once it reaches the root directory of the repo
itself.

Add a test that verifies that we are indeed not applying macros from
subdirectories. Previous to these changes, the test would've failed.