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    commit: improve the files field of changelog for merges · 99ebde4fec99
    sliquister authored
    Currently, the files list of merge commits repeats all the deletions
    (either actual deletions, or files that got renamed) that happened
    between base and p2 of the merge. If p2 is the main branch, the list
    can easily be much bigger than the change being merged.
    
    This results in various problems worth improving:
    - changelog is bigger than necessary
    - `hg log directory` lists many unrelated merge commits, and `hg log
      -v -r commit` frequently fills multiple screens worth of files
    - it possibly slows down adjustlinkrev, by forcing it to read more
      manifests, and that function can certainly be a bottleneck
    - the server side of pulls can waste a lot of time simply opening the
      filelogs for pointless files (the constant factors for opening even
      a tiny filelog is apparently pretty bad)
    
    So stop listing such files as described in the code.  Impacted merge
    commits and their descendants get a different hash than they would
    have without this. This doesn't seem problematic, except for
    convert. The previous commit helped with that in the hg->hg case (but
    if you do svn->hg twice from scratch, hashes can still change).
    
    The rest of the description is numbers. I don't have much to report,
    because recreating the files list of existing repositories is not
    easy:
    - debugupgradeformat and bundle/unbundle don't recreate the list
    - export/import tends to choke quickly applying patches or on
      description that contain diffs,
    - merge commits from the convert extension don't have the right files
      list for reasons orthogonal to the current commit
    - replaying the merge with hg update/hg merge/hg revert --all/hg
      commit can end up failing in hg revert
    - I wasn't sure that using debugsetparents + debugrebuilddirstate
      would really build the right thing
    
    I measured commit time before and after this change, in a case with no
    files filtered out, several files filtered out (no difference) and 5k
    files filtered out (+1% time).
    
    Recreating the 100 more recent merges in a private repo, the
    concatenated uncompressed files lists goes from 1.12MB to
    0.52MB. Excluding 3 merges that are not representative, then the size
    goes from 570k to 15k.
    I converted part of mozilla-central, and observed file list shrinking
    quite a bit too, starting at the very first merge, 733641d9feaf, going
    from 550 files to 10 files (although they have relatively few merges,
    so they probably wouldn't care).
    
    Differential Revision: https://phab.mercurial-scm.org/D6613
    99ebde4fec99