relnotes: document automatically setting `HGPLAIN=1` for external hooks Differential Revision: https://phab.mercurial-scm.org/D9629

cext: isolate hash size in the revlog handling in a single place Differential Revision: https://phab.mercurial-scm.org/D9450

rust: document how to enable debug information in optimized builds This is particularly useful when profiling. Differential Revision: https://phab.mercurial-scm.org/D9625

filemerge: add support for the new "mergediff" marker style to premerge This adds support for the new "mergediff" conflict marker style to "merge-tools.<tool>.premerge" (the new value is called "keep-mergediff"). Differential Revision: https://phab.mercurial-scm.org/D9606

test-narrow: mitigate the flakyness of test-narrow-shallow The actual bug still need fixing, but in the meantime the flakyness is wasting everybody's time. Differential Revision: https://phab.mercurial-scm.org/D9593

hook: set `HGPLAIN=1` for external hooks External hooks will probably turn around and run hg commands, so this seems like a reasonable convenience for people who miss it in the documentation. There are no test changes because `printenv.py` filters out anything without a "HG_" prefix. It wouldn't be a useful test anyway, because this is already inherited from the test environment. Differential Revision: https://phab.mercurial-scm.org/D9605

setup: exclude the git extension from py2 builds This can't be built on Windows with the py2 compiler, and while old versions can be installed via pip on Linux, I can't get the tests to run (even with py3.8) using pygit2 0.28.2. Some manually run commands work, and others spew stack traces that don't occur with the current 1.4.0 release using py3. Differential Revision: https://phab.mercurial-scm.org/D9604

rust: replace most "operation" structs with functions The hg-core crate has a partially-formed concept of "operation", represented as structs with constructors and a `run` method. Each struct’s contructor takes different parameters, and each `run` has a different return type. Constructors typically don’t do much more than store parameters for `run` to access them. There was a comment about adding an `Operation` trait when the language supports expressing something so general, but it’s hard to imagine how operations with such different APIs could be used in a generic context. This commit starts removing the concept of "operation", since those are pretty much just functions. Differential Revision: https://phab.mercurial-scm.org/D9595

rust: change &PathBuf parameters to &Path This is just as useful in function bodies, and a less strict requirement for callers. Differential Revision: https://phab.mercurial-scm.org/D9594

extdiff: add --from/--to and deprecate -r, as was done for `hg diff` I recently replaced `hg diff`'s `-r` flag by `--from` and `--to` and then deprecated the `-r` flag. This patch repeats that for `hg extdiff`. Differential Revision: https://phab.mercurial-scm.org/D9624