ref namespaces: documentation

ref namespaces: documentation

Document the namespace mechanism in a new gitnamespaces(7) page.
Reference it from receive-pack and upload-pack.

Document the new --namespace option and GIT_NAMESPACE environment
variable in git(1), and reference gitnamespaces(7).

Add a sample Apache configuration to http-backend(1) to support
namespaced repositories, and reference gitnamespaces(7).

Signed-off-by: Josh Triplett <redacted>
Signed-off-by: Jamey Sharp <redacted>
Signed-off-by: Junio C Hamano <redacted>