for a given site.
This patch uses the fact that getFolderByServerRelativeUrl API returns
site's root folder if used with empty path ('').
Hopefully this isn't otherwise context-dependent, so that it doesn't
return other folders in some unknown circumstances.

More correct would be instead to use the site's root path directly,
like '/sites/mysite'. But I don't know a reliable way to filter the
API part from the base URI.

To make use of the corrected SharePoint repository root id, the patch
also redirects all cases where "/" object is queried, to session's
getRootFolder().

libcmis part of the patch will hopefully soon be merged upstream.

Change-Id: I0f3b9930748de3fe4763eb27721739f097b9892d
Reviewed-on: https://gerrit.libreoffice.org/27987Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>