wrap shared_repository global in get/set accessors

wrap shared_repository global in get/set accessors

It would be useful to control access to the global
shared_repository, so that we can lazily load its config.
The first step to doing so is to make sure all access
goes through a set of functions.

This step is purely mechanical, and should result in no
change of behavior.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>