t5401: speed up creation of many branches

One of the tests in t5401 creates a bunch of branches by calling git-branch(1) for every one of them. This is quite inefficient and takes a comparatively long time even on Unix systems where spawning processes is comparatively fast. Refactor it to instead use git-update-ref(1), which leads to an almost 10-fold speedup: ``` Benchmark 1: ./t5401-update-hooks.sh (rev = HEAD) Time (mean ± σ): 983.2 ms ± 97.6 ms [User: 328.8 ms, System: 679.2 ms] Range (min … max): 882.9 ms … 1078.0 ms 3 runs Benchmark 2: ./t5401-update-hooks.sh (rev = HEAD~) Time (mean ± σ): 9.312 s ± 0.398 s [User: 2.766 s, System: 6.617 s] Range (min … max): 8.885 s … 9.674 s 3 runs Summary ./t5401-update-hooks.sh (rev = HEAD) ran 9.47 ± 1.02 times faster than ./t5401-update-hooks.sh (rev = HEAD~) ``` Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>