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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

t/t5401-update-hooks.sh

@@ -133,10 +133,8 @@ test_expect_success 'pre-receive hook that forgets to read its input' '
 	EOF
 	rm -f victim.git/hooks/update victim.git/hooks/post-update &&
 
-	for v in $(test_seq 100 999)
-	do
-		git branch branch_$v main || return
-	done &&
+	printf "create refs/heads/branch_%d main\n" $(test_seq 100 999) >input &&
+	git update-ref --stdin <input &&
 	git push ./victim.git "+refs/heads/*:refs/heads/*"
 '