test-check-code: prevent from adding Python modules shadowed by ancient C

diffhelper: rename module to avoid conflicts with ancient C module (issue5846) Historically we had had C extensions in mercurial/, which shadows the pure Python modules of the same name forever unless we do clean build/install. I'm sloppy to think about new name, so just dropped the "s".

diffhelpers: backport 9e40bc4c1bde from C implementation 9e40bc4c1bde just says "harden testhunk." I don't think this would be the case, but it makes some sense to avoid negative index.

version: make parser more robust for rc variants and ill-formed strings

phabricator: specify some metadata compatibly with arc Differential Revision: https://phab.mercurial-scm.org/D3414

diff: restore original color scheme for worddiff I'm not young. "red dim" is nearly invisible, and "red" vs "red dim" is too subtle to see difference. And, "underline" is necessary to highlight whitespace changes.

test-fix: fix use of 'f --newer' to check that foo.whole is not updated Here we want to assert not 'mtime(foo.whole.orig) < mtime(foo.whole)'. The condition has to be inverted since 'f --newer' says "newer" if the mtimes match. Alternatively, we could insert 'sleep 2' before 'cp' to ensure that foo.whole.orig is newer than foo.whole, but a fewer sleeps should be better.

context: translate FilteredIndex/LookupError at repo[changeid] (API) This partially backs out ecd3f6909184. It seems layering violation for repo[changeid] to raise storage-level exceptions transparently. Otherwise, we would have to rewrite callers to catch all of them. try: repo[rev_or_node] except (error.RepoLookupError, error.FilteredIndexError, error.FilteredLookupError): pass This would also fix filectx._changectx(), which catches FilteredRepoLookupError to fall back to the unfiltered path.