This adds support for shallow cloning. The CloneCommand and the
FetchCommand now have the new options --depth, --shallow-since and
--shallow-exclude to tell the server that the client doesn't want to
download the complete history.
Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=475615
Change-Id: I8f113bed25dd6df64f2f95de6a59d4675ab8a903