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