Merge branch 'sg/line-log-merge-optim'

"git log -L..." compared trees of multiple parents with the tree of the
merge result in an unnecessarily inefficient way.

* sg/line-log-merge-optim:
  line-log: simplify condition checking for merge commits
  line-log: initialize diff queue in process_ranges_ordinary_commit()
  line-log: get rid of the parents array in process_ranges_merge_commit()
  line-log: avoid unnecessary tree diffs when processing merge commits