import-checker: ignore nested imports

contrib: add an import checker This checks for cycles in the module graph and verifies that imports of stdlib modules are not on the same line as relative imports of mercurial modules.

makememctx: move from patch to context to break import cycle

cleanup: move stdlib imports to their own import statement There are a few warnings still produced by my import checker, but those are false positives produced by modules that share a name with stdlib modules.

pathutil: tease out a new library to break an import cycle from canonpath use

subsettable: move from repoview to branchmap, the only place it's used This is a step towards breaking an import cycle between revset and repoview. Import cycles happened to work in Python 2 with implicit relative imports, but breaks on Python 3 when we start using explicit relative imports via 2to3 rewrite rules.

obsolete: add __eq__ and __hash__ to marker to make set() deduplication work

obsolete: do not accept duplicated marker during exchange Before this patch, duplicated obsolescence markers could slip into an obstore if the bookmark was unknown locally and duplicated in the incoming obsolescence stream. Existing duplicate markers will not be automatically removed but they'll stop propagating. Having a few duplicated markers is harmless and people have been warned evolution is <blink>experimental</blink> anyway.

Merge with stable.

obsolete: stop doing membership test on list According to the Surgeon General, computer should not use list for membership testing because of the risk of being slow.