stringutil: extract helper function that splits stringmatcher() pattern

grep: extract main search loop as searcher method Still displayer part is in commands.grep(), the core grep logic is now reusable. I'll revisit the displayer stuff later since it will be another long series.

scmutil: move walkchangerevs() from cmdutil It's no longer a command-level function, but a pure helper to walk revisions in a windowed way. This change will help eliminate reverse dependency of revset.py -> grep.py -> cmdutil.py in future patches.

grep: extract public function to register file to be skipped The main grep loop will be extracted to a searcher method, but this skipping condition depends on the result of display() function.

posix: avoid a leaked file descriptor in a unix domain socket exception case Differential Revision: https://phab.mercurial-scm.org/D9206

posix: use context managers in a couple of places Differential Revision: https://phab.mercurial-scm.org/D9205

record: when backing up, avoid generating very long filenames If the original file's path is longer than the individual filename maximum length (256 on Linux, I believe?), then this mechanism of "replace slashes with underscores" causes an error. Now, we'll produce just the "basename" of the file, plus some stuff to ensure it's unique. This can be potentially confusing for users if there's a file with the same name in multiple directories, but I suspect that this is better than just breaking. Example: `<reporoot>/a/long/path/to/somefile.txt` used to be backed up as `<reporoot>/.hg/record-backups/a_long_path_to_somefile.txt.abcdefgh`, it will now be backed up as `<reporoot>/.hg/record-backups/somefile.txt.abcdefgh` We could do the naive thing (what we were doing before) and have it to doing something with either subdirectories (`<backuproot>/a/long/path/to/somefile.txt.abcdefgh` or minimize #dirs with `<backuproot>/a_long_path/to_somefile.txt.abcdefgh`), prefix-truncated paths (such as `<backuproot>/__ath_to_somefile.txt.abcdefgh`, where that `__` elides enough to get us under 255 chars (counting the +9 we need to add!)), or hash-of-dirname (`<backuproot>/<sha1sum_of_dirname>/somefile.txt.abcdefgh`), but ultimately every option felt over engineered and that it would be more likely to cause problems than it would be to solve any, especially if it was conditional on directory length. Differential Revision: https://phab.mercurial-scm.org/D9207

mergestate: add `allextras()` to get all extras `extras()` can only be used for getting extra for a file. However at couple of places in code, we wanted to iterate over all the extras stored with the mergestate and they were accessing the private `_stateextras`. We add a new function for this. Differential Revision: https://phab.mercurial-scm.org/D9190

mergestate: document `o` merge record state in _mergestate_base docs _mergestate_base documentation serves as a nice documentation for mergestate. This also documents known merge records and known merge record states. I missed adding `o` state to it when I introduced it. Let's add it now. Differential Revision: https://phab.mercurial-scm.org/D9156

fix: update commit hash references in the new commits Differential Revision: https://phab.mercurial-scm.org/D9183

absorb: update commit hash references in the new commits Differential Revision: https://phab.mercurial-scm.org/D9182

rewriteutil: handle dropped commits when updating description hashes In looking to leverage this with the absorb extension, the old -> new mapping there allows the new value to be None. We could filter that out and not pass it to this method, but it seems worth a message to the user. (I wonder if these should be an info or warning, because it's unlikely people are using `-v` regularly.) Differential Revision: https://phab.mercurial-scm.org/D9181

dirstate-tree: move a conditional in an explicit boolean This will help readability a bit and make the next change simpler to read. Differential Revision: https://phab.mercurial-scm.org/D9202

rust: cleanup some white space in a dock They seems to have sneaked there somehow. Differential Revision: https://phab.mercurial-scm.org/D9201

commit: pass ChangingFiles object as argument to _process_files Instead of returning it, we pass it as an argument. This makes the whole if-else in `_prepare_files` a bit simpler. Else each if-else branch was creating the object. Differential Revision: https://phab.mercurial-scm.org/D9194

commit: pass mergestate into `_process_files` instead of re-reading it Differential Revision: https://phab.mercurial-scm.org/D9193

commit: move salvaged calculation a bit earlier in the function This helps us initialize mergestate before and now we can pass it into `_process_files()` instead of re-reading it there. Differential Revision: https://phab.mercurial-scm.org/D9192

commit: refactor salvage calculation to a different function Differential Revision: https://phab.mercurial-scm.org/D9191

tests: run test-copies-chain-merge.t also with copies in changesets We have these tests already and it seems like a waste to not run them in the changesets case. The biggest differences stem from `hg log --follow` not working with copies stored in the changeset extras. Differential Revision: https://phab.mercurial-scm.org/D9173

tests: add test of copies suggested by Pierre-Yves on D9159 Differential Revision: https://phab.mercurial-scm.org/D9171

tests: update test-copies-chain-merge.t to not use empty files Merging empty files is not very interesting or realistic. Differential Revision: https://phab.mercurial-scm.org/D9172

revset: fix sorting key of wdir revision It would go wrong on Python 2, and would crash on Python 3.

grep: move prep() to grepsearcher class

grep: move readfile() to grepsearcher class

grep: move getbody() to grepsearcher class

grep: add stub class that maintains cache and states of grep operation Prepares for extracting stateful functions from commands.grep().

grep: move match and diff logic to new module commands.grep() has lots of functions and classes. Let's split it into reusable components so we can leverage them to implement a revset predicate for 'hg grep --diff'. I want to do 'hg log -r "diff(pattern)"'.

grep: explicitly pass regexp to closure functions These functions will be extracted to new module.

upgrade: improve documentation of matchrevlog() It was not clear what selected for cloning meant. I updated with an extra line of description.

automation: support running against Python 3.9 We just added CPython 3.9.0 to the automation environment. Let's support actions targeting it. One side-effect of this change is that we'll produce Windows wheels for Python 3.9 by default. This is desirable. Differential Revision: https://phab.mercurial-scm.org/D9189