Stop traversing the filesystem at the Windows directory root.  We were
calculating the filesystem root for the given directory to create, and
walking up the filesystem hierarchy.  We intended to stop when the
traversal path length is equal to the root path length (ie, stopping at
the root, since no path may be shorter than the root path).

However, on Windows, the root path may be specified in two different
ways, as either `Z:` or `Z:\`, where `Z:` is the current drive letter.
`git_path_dirname_r` returns the path _without_ a trailing slash, even
for the Windows root.  As a result, during traversal, we need to test
that the traversal path is _less than or equal to_ the root path length
to determine if we've hit the root to ensure that we stop when our
traversal path is `Z:` and our calculated root path was `Z:\`.

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.

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

Update internal usage to use the `git_reference` names for constants.

Use the new object_type enumeration names within the codebase.

Test updated symbolic link creation on Windows.  Ensure that we emulate
Git for Windows behavior.  Ensure that when `core.symlinks=true` is set
in a global configuration that new repositories are created without a
`core.symlinks` setting, and that when `core.symlinks` is unset that
`core.symlinks=false` in set in the repository.  Further ensure that
checkout honors the expected `core.symlinks` defaults on Windows.

Provide a function that allows tests to set up a bespoke global
configuration path.

Ensure that `core.symlinks` is set correctly.  By default, it is unset,
but it is explicitly set to `false` if the platform was detected to not
support symlinks during repository initialization.

C++ style comment ("//") are not specified by the ISO C90 standard and
thus do not conform to it. While libgit2 aims to conform to C90, we did
not enforce it until now, which is why quite a lot of these
non-conforming comments have snuck into our codebase. Do a tree-wide
conversion of all C++ style comments to the supported C style comments
to allow us enforcing strict C90 compliance in a later commit.