The second part of the Share Safe Plan is to share source repo config also.
This patch adds logic to load the source repo .hg/hgrc if we are in share safe
mode. On unshare, we copy and prepend source config to current repo so that
config which was shared is persisted.
A test is added to show that now if we enable a hook on the source repo, that
also runs on the shared repositories.
API change as a new optional argument sharedvfs added to localrepo.loadhgrc()