scmutil.filecache: support watching over multiple files

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.