Merge branch 'jc/lock-report-on-error'

Git 2.11 had a minor regression in "merge --ff-only" that competed
with another process that simultanously attempted to update the
index. We used to explain what went wrong with an error message,
but the new code silently failed.  The error message has been
resurrected.

* jc/lock-report-on-error:
  lockfile: LOCK_REPORT_ON_ERROR
  hold_locked_index(): align error handling with hold_lockfile_for_update()
  wt-status: implement opportunisitc index update correctly

diff --cc builtin/clone.c
Simple merge

diff --cc builtin/commit.c
Simple merge

diff --cc merge-recursive.c
Simple merge

diff --cc sequencer.c
Simple merge