Test to ensure that we can create a repository at the filesystem
root. Introduces a new test environment variable,
`GITTEST_INVASIVE_FILESYSTEM` for tests that do terrible things like
escaping the clar sandbox and writing to the root directory. It is
expected that the CI builds will enable this but that normal people
would not want this.
Remove the hook symlink from the test resources, so that we can
have a source tree that is easy to zip up and copy around on systems
that don't support symlinks. Create it dynamically at test execution
instead.
Teach git_repository_init_ext to use relative paths for the gitlink
to the work directory. This is used when creating a sub repository
where the sub repository resides in the parent repository's
.git directory.
