scmutil: introduce a filecacheentry that can watch multiple paths

scmutil: rename filecacheentry to filecachesubentry Upcoming patches will allow the file cache to watch over multiple files, and call the decorated function again if any of the files change. The particular use case for this is the bookmark store, which needs to be invalidated if either .hg/bookmarks or .hg/bookmarks.current changes. (This doesn't currently happen, which is a bug. This bug will also be fixed in upcoming patches.)

scmutil.filecacheentry: make stat argument to constructor mandatory There's no real upside to making stat optional -- this constructor has just two callers.

test-filecache.py: add markers to the output for each event Previously it was possible that a different, incorrect set of events might print out 'creating' the same number of times.

test-filecache.py: make setbeforeget test clearer '0' and 'None' as outputs tripped me up. Make the distinction between values set externally and values computed by calling the decorated function clearer.

test-module-imports.t: new test to use the import cycle detector

import-checker: try a little harder to show fewer cycles This makes sure that all cycles begin with the lexicographically first module, so that we're less likely to show overlapping cycles in the final analysis.

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.