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.

Update the test cases to check the `git_repository_open` return code.

Git has supported repository format version 1 for some time.  This
format is just like version 0, but it supports extensions.
Implementations must reject extensions that they don't support.

Add support for this format version and reject any extensions but
extensions.noop, which is the only extension we currently support.

While we're at it, also clean up an error message.

Regression introduced in commit dde6d9c706bf1ecab545da55ab874a016587af1f.

This issue causes lots of crashes in TortoiseGit.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

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.

We've got reports that users are unable to open repos when their global
configuration ("~/.gitconfig") is a symlink. Add a test to verify that
we are in fact able to do so as expected.