Merge branch 'jc/merge' into next

Merge branch 'jc/merge' into next

"git merge FETCH_HEAD" learned that the previous "git fetch" could
be to create an Octopus merge, i.e. recording multiple branches
that are not marked as "not-for-merge"; this allows us to lose an
old style invocation "git merge <msg> HEAD $commits..." in the
implementation of "git pull" script; the old style syntax can now
be deprecated.

* jc/merge:
  merge: deprecate 'git merge <message> HEAD <commit>' syntax
  merge: handle FETCH_HEAD internally
  merge: decide if we auto-generate the message early in collect_parents()
  merge: make collect_parents() auto-generate the merge message
  merge: extract prepare_merge_message() logic out
  merge: narrow scope of merge_names
  merge: split reduce_parents() out of collect_parents()
  merge: clarify collect_parents() logic
  merge: small leakfix and code simplification
  merge: do not check argc to determine number of remote heads
  merge: clarify "pulling into void" special case
  t5520: test pulling an octopus into an unborn branch
  t5520: style fixes
  merge: simplify code flow
  merge: test the top-level merge driver