merge-ort: restart merge with cached renames to reduce process entry cost

The merge algorithm mostly consists of the following three functions: collect_merge_info() detect_and_process_renames() process_entries() Prior to the trivial directory resolution optimization of the last half dozen commits, process_entries() was consistently the slowest, followed by collect_merge_info(), then detect_and_process_renames(). When the trivial directory resolution applies, it often dramatically decreases the amount of time spent in the two slower functions. Looking at the performance results in the previous commit, the trivial directory resolution optimization helps amazingly well when there are no relevant renames. It also helps really well when reapplying a long series of linear commits (such as in a rebase or cherry-pick), since the relevant renames may well be cached from the first reapplied commit. But when there are any relevant renames that are not cached (represented by the just-one-mega testcase), then the optimization does not help at all. Often, I noticed that when the optimization does not apply, it is because there are a handful of relevant sources -- maybe even only one. It felt frustrating to need to recurse into potentially hundreds or even thousands of directories just for a single rename, but it was needed for correctness. However, staring at this list of functions and noticing that process_entries() is the most expensive and knowing I could avoid it if I had cached renames suggested a simple idea: change collect_merge_info() detect_and_process_renames() process_entries() into collect_merge_info() detect_and_process_renames() <cache all the renames, and restart> collect_merge_info() detect_and_process_renames() process_entries() This may seem odd and look like more work. However, note that although we run collect_merge_info() twice, the second time we get to employ trivial directory resolves, which makes it much faster, so the increased time in collect_merge_info() is small. While we run detect_and_process_renames() again, all renames are cached so it's nearly a no-op (we don't call into diffcore_rename_extended() but we do have a little bit of data structure checking and fixing up). And the big payoff comes from the fact that process_entries(), will be much faster due to having far fewer entries to process. This restarting only makes sense if we can save recursing into enough directories to make it worth our while. Introduce a simple heuristic to guide this. Note that this heuristic uses a "wanted_factor" that I have virtually no actual real world data for, just some back-of-the-envelope quasi-scientific calculations that I included in some comments and then plucked a simple round number out of thin air. It could be that tweaking this number to make it either higher or lower improves the optimization. (There's slightly more here; when I first introduced this optimization, I used a factor of 10, because I was completely confident it was big enough to not cause slowdowns in special cases. I was certain it was higher than needed. Several months later, I added the rough calculations which make me think the optimal number is close to 2; but instead of pushing to the limit, I just bumped it to 3 to reduce the risk that there are special cases where this optimization can result in slowing down the code a little. If the ratio of path counts is below 3, we probably will only see minor performance improvements at best anyway.) Also, note that while the diffstat looks kind of long (nearly 100 lines), more than half of it is in two comments explaining how things work. For the testcases mentioned in commit 557ac0350d ("merge-ort: begin performance work; instrument with trace2_region_* calls", 2020-10-28), this change improves the performance as follows: Before After no-renames: 205.1 ms ± 3.8 ms 204.2 ms ± 3.0 ms mega-renames: 1.564 s ± 0.010 s 1.076 s ± 0.015 s just-one-mega: 479.5 ms ± 3.9 ms 364.1 ms ± 7.0 ms Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>