diff: when looking for diff configs, support a configurable prefix In a future commit, I want to make it possible to have the diff options pulled from (as an example) `commands.commit.interactive.ignorews`; previously we only supported this for customizable sections (so this would have needed a `commit-interactive` section and been named `commit-interactive.ignorews`, which felt a bit weird. Differential Revision: https://phab.mercurial-scm.org/D5833

config: extract diff-related coreconfigitem()s to a helper method We already have 'annotate' and 'diff' that use the same set of options, and I want to add more in a followup commit, so I'm attempting to reduce maintenance burden and duplication by making it possible to register all of them at once. Differential Revision: https://phab.mercurial-scm.org/D5832

commit: ignore diff whitespace settings when doing `commit -i` (issue5839) Previously, we respected options like `diff.ignoreblanklines` and `diff.ignorews`. This can cause problems when the user is attempting to actually commit the blank line change. Specifically, the split extension can get into an infinite loop because it detects that the working copy is not clean, but when we get the diff we don't see the changes, so it just skips popping up the chunk selection flow, saying there's no changes to record. These options are primarily meant for viewing diffs; it is highly unlikely that someone is actually intending to add extraneous whitespace and have it ignored if they attempt to interactively commit (but *not* ignored if they non-interactively commit). Differential Revision: https://phab.mercurial-scm.org/D5744

tests: add syntax warnings on Python 3.8 in python-zstandard These are warnings from upstream test code. Let's just acknowledge them for now. These should go away in a future python-zstandard release :) Differential Revision: https://phab.mercurial-scm.org/D5846

check-py3-compat: manually format and print warnings The warnings mechanism may print to stderr on Python 3. Independent buffering of stdout and stderr can lead to warnings output not being printed properly. This commit traps warnings when executing individual files and prints warnings to stdout so output is deterministic. Differential Revision: https://phab.mercurial-scm.org/D5845

check-py3-compat: provide filename to ast.parse() This ensures any warning/error messages print a valid filename instead of potentially '<unknown>'. Differential Revision: https://phab.mercurial-scm.org/D5844

doc: escape backslash This avoids a SyntaxWarning on Python 3.8 due to invalid \ escape. Differential Revision: https://phab.mercurial-scm.org/D5843

testparseutil: escape backslash in docstring This is funky. This inline Python code is part of a docstring, which means the string is interpreted first. So any backslashes need double escaping. So even though this is already a br'', we still need to escape. Differential Revision: https://phab.mercurial-scm.org/D5842