rebase: change the default backend from "am" to "merge"

The am-backend drops information and thus limits what we can do: * lack of full tree information from the original commits means we cannot do directory rename detection and warn users that they might want to move some of their new files that they placed in old directories to prevent their becoming orphaned.[1] * reduction in context from only having a few lines beyond those changed means that when context lines are non-unique we can apply patches incorrectly.[2] * lack of access to original commits means that conflict marker annotation has less information available. * the am backend has safety problems with an ill-timed interrupt. Also, the merge/interactive backend have far more abilities, appear to currently have a slight performance advantage[3] and have room for more optimizations than the am backend[4] (and work is underway to take advantage of some of those possibilities). [1] https://lore.kernel.org/git/xmqqh8jeh1id.fsf@gitster-ct.c.googlers.com/ [2] https://lore.kernel.org/git/CABPp-BGiu2nVMQY_t-rnFR5GQUz_ipyEE8oDocKeO+h+t4Mn4A@mail.gmail.com/ [3] https://public-inbox.org/git/CABPp-BF=ev03WgODk6TMQmuNoatg2kiEe5DR__gJ0OTVqHSnfQ@mail.gmail.com/ [4] https://lore.kernel.org/git/CABPp-BGh7yW69QwxQb13K0HM38NKmQif3A6C6UULEKYnkEJ5vA@mail.gmail.com/ Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>