mergesort: tighten merge loop

mergesort: tighten merge loop

llist_merge() has special inner loops for taking elements from either of
the two lists to merge.  That helps consistently preferring one over the
other, for stability.  Merge the loops, swap the lists when the other
one has the next element for the result and keep track on which one to
prefer on equality.  This results in shorter code and object text:

Before:
__TEXT	__DATA	__OBJC	others	dec	hex
412	0	0	3441	3853	f0d	mergesort.o

With this patch:
__TEXT	__DATA	__OBJC	others	dec	hex
352	0	0	3516	3868	f1c	mergesort.o

Performance doesn't get worse:

Before:
0071.12: llist_mergesort() unsorted    0.24(0.22+0.01)
0071.14: llist_mergesort() sorted      0.12(0.10+0.01)
0071.16: llist_mergesort() reversed    0.12(0.10+0.01)

Benchmark 1: t/helper/test-tool mergesort test
  Time (mean ± σ):     109.2 ms ±   0.2 ms    [User: 107.5 ms, System: 1.1 ms]
  Range (min … max):   108.9 ms … 109.6 ms    27 runs

With this patch:
0071.12: llist_mergesort() unsorted    0.24(0.22+0.01)
0071.14: llist_mergesort() sorted      0.12(0.10+0.01)
0071.16: llist_mergesort() reversed    0.12(0.10+0.01)

Benchmark 1: t/helper/test-tool mergesort test
  Time (mean ± σ):     108.4 ms ±   0.2 ms    [User: 106.7 ms, System: 1.2 ms]
  Range (min … max):   108.0 ms … 108.8 ms    27 runs

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>