Change to reading objects via custom stream wrapper

For new stream URL use 'git-object://verb/url' where 'verb' is the
expected object type ("blob", "commit" or "tree") and 'url' is a valid
location to read directly from.
New URL is served from `Repository::getContentURL()`

Since SHA1 hashes can be considered universal there is no need to know
which repository to query.  Use `Repository::walk('getContentURL')` to
call all repositories in memory.
Names got swapped by accident
Documentation
Remove references to `$this->repository`
Must specify object type (verb) as part of content URLs
Allow multiple parents per commit, accept first as dominant