tests: extend check-commit self-tests This now contains both positive and negative tests for almost all the check-commit rules.

encoding: handle UTF-16 internal limit with fromutf8b (issue5031) Default builds of Python have a Unicode type that isn't actually full Unicode but UTF-16, which encodes non-BMP codepoints to a pair of BMP codepoints with surrogate escaping. Since our UTF-8b hack escaping uses a plane that overlaps with the UTF-16 escaping system, this gets extra complicated. In addition, unichr() for codepoints greater than U+FFFF may not work either. This changes the code to reuse getutf8char to walk the byte string, so we only rely on Python for unpacking our U+DCxx characters.

bmstore: add handling of the active bookmark This further centralizes the handling of bookmark storage, and will help get some lingering bookmarks business out of localrepo. Right now, this change implies reading of the active bookmark to also imply reading all bookmarks from disk - for users with many many bookmarks this may be a measurable performance hit. In that case, we should migrate bmstore to be able to lazy-read its properties from disk rather than having to eagerly read them, but I decided to avoid doing that to try and avoid some potentially complicated filecache decorator issues. This doesn't move the logic for writing the active bookmark into a transaction, though that is probably the correct next step. Since the API probably needs to morph a little more, I didn't bother marking bookmarks.{activate,deactivate} as deprecated yet.