context: add instabilities() method to basefilectx This method is now used in webutils.commonentry(), which adds common data items (commit hash, author, date, etc) for rendering changesets in hgweb. Usually, commonentry() is given a changectx as ctx; but in views related to files (e.g. file view, diff, annotate) it's replaced by a filectx, so the latter also needs to have instabilities() method.

run-test: drop 'execfile' usage for 'common-pattern.py' file This is required for Python 3.

run-test: use byte for 'common-pattern.py' path This is required for Python 3.