builtin/am: use apply API in run_apply()
This replaces run_apply() implementation with a new one that uses the apply API that has been previously prepared in apply.c and apply.h. This shoud improve performance a lot in certain cases. As the previous implementation was creating a new `git apply` process to apply each patch, it could be slow on systems like Windows where it is costly to create new processes. Also the new `git apply` process had to read the index from disk, and when the process was done the calling process discarded its own index and read back from disk the new index that had been created by the `git apply` process. This could be very inefficient with big repositories that have big index files, especially when the system decided that it was a good idea to run the `git apply` processes on a different processor core. Also eliminating index reads enables further performance improvements by using: `git update-index --split-index` For example here is a benchmark of a multi hundred commit rebase on the Linux kernel on a Debian laptop with SSD: command: git rebase --onto 1993b17 52bef0c 29dde7c Vanilla "next" without split index: 1m54.953s Vanilla "next" with split index: 1m22.476s This series on top of "next" without split index: 1m12.034s This series on top of "next" with split index: 0m15.678s (using branch "next" from mid April 2016.) Benchmarked-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: NChristian Couder <chriscool@tuxfamily.org> Signed-off-by: NJunio C Hamano <gitster@pobox.com>
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