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