configitems: move the default values to the registrar decorator The exthelper refactoring of configitems stopped registering dynamicdefault for everything, which in turn causes a lot of warnings about mismatched defaults. There are a few stragglers- I didn't fix topic-mode because I wasn't sure what the default should be, and the alias for odiff was explicitly given a default for compatibility.

topic: make revsets like foo#stack[-1] work too We remove base commit when n < 0 because it's confusing to see base (which technically isn't a commit inside the stack) at both 0 and -n.