line-log: avoid unnecessary tree diffs when processing merge commits

line-log: avoid unnecessary tree diffs when processing merge commits

In process_ranges_merge_commit(), the line-level log first creates an
array of diff queues by iterating over all parents of a merge commit
and computing a tree diff for each.  Then in a second loop it iterates
over those diff queues, and if it finds that none of the interesting
paths were modified in one of them, then it will return early.  This
means that when none of the interesting paths were modified between a
merge and its first parent, then the tree diff between the merge and
its second (Nth...) parent was computed in vain.

Unify these two loops, so when it iterates over all parents of a merge
commit, then it first computes the tree diff between the merge and
that particular parent and then processes the resulting diff queue
right away.  This way we can spare some tree diff computing, thereby
speeding up line-level log in repositories with mergy history:

  # git.git, 25.8% of commits are merges:
  Benchmark 1: ./git_v2.51.0 -C ~/src/git log -L:'lookup_commit(':commit.c v2.51.0
    Time (mean ± σ):      1.001 s ±  0.009 s    [User: 0.906 s, System: 0.095 s]
    Range (min … max):    0.991 s …  1.023 s    10 runs

  Benchmark 2: ./git -C ~/src/git log -L:'lookup_commit(':commit.c v2.51.0
    Time (mean ± σ):     445.5 ms ±   3.4 ms    [User: 358.8 ms, System: 84.3 ms]
    Range (min … max):   440.1 ms … 450.3 ms    10 runs

  Summary
    './git -C ~/src/git log -L:'lookup_commit(':commit.c v2.51.0' ran
      2.25 ± 0.03 times faster than './git_v2.51.0 -C ~/src/git log -L:'lookup_commit(':commit.c v2.51.0'

  # linux.git, 7.5% of commits are merges:
  Benchmark 1: ./git_v2.51.0 -C ~/src/linux.git log -L:build_restore_work_registers:arch/mips/mm/tlbex.c v6.16
    Time (mean ± σ):      3.246 s ±  0.007 s    [User: 2.835 s, System: 0.409 s]
    Range (min … max):    3.232 s …  3.255 s    10 runs

  Benchmark 2: ./git -C ~/src/linux.git log -L:build_restore_work_registers:arch/mips/mm/tlbex.c v6.16
    Time (mean ± σ):      2.467 s ±  0.014 s    [User: 2.113 s, System: 0.353 s]
    Range (min … max):    2.455 s …  2.505 s    10 runs

  Summary
    './git -C ~/src/linux.git log -L:build_restore_work_registers:arch/mips/mm/tlbex.c v6.16' ran
      1.32 ± 0.01 times faster than './git_v2.51.0 -C ~/src/linux.git log -L:build_restore_work_registers:arch/mips/mm/tlbex.c v6.16'

And since now each iteration computes a tree diff and processes its
result, there is no reason to store the diff queues for each merge
parent anymore, so replace that diff queue array with a loop-local
diff queue variable.  With this change the static free_diffqueues()
helper function in 'line-log.c' has no more callers left, remove it.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>