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