remotefilelog: avoid accessing repo instance after dispatch
Upstream commit c5e6c1ba1c79 (hg: don't reuse repo instance after
unshare(), 2018-09-12) poisoned the repo instance after
unshare(). That made hg unshare fail with remotefilelog because we
tried to close the fileserverclient after dispatch by accessing it via
the repo. This patch fixes that by storing the reference to the
fileserverclient at the beginning of dispatch.
Differential Revision: https://phab.mercurial-scm.org/D5246