Merge branch 'jc/merge-reduce-parents-early' into maint

Merge branch 'jc/merge-reduce-parents-early' into maint

Octopus merge strategy did not reduce heads that are recorded in the final
commit correctly.

By Junio C Hamano (4) and Michał Kiedrowicz (1)
* jc/merge-reduce-parents-early:
  fmt-merge-msg: discard needless merge parents
  builtin/merge.c: reduce parents early
  builtin/merge.c: collect other parents early
  builtin/merge.c: remove "remoteheads" global variable
  merge tests: octopus with redundant parents