hgweb: use ui._unset to prevent a warning in configitems

dispatch: fix typo suggestion for disabled extension If the matching command lives in an in-tree extension (which is all we scan for), and the user has disabled that extension with "extensions.<name>=!", we were not finding it, because the path in _disabledextensions was the empty string. If the user had set "extensions.<name>=!<valid path>" it would work, so it seems like just a mistake that it didn't work.

tests: add tests for typoed commands This includes one test showing how disabling a command with e.g. "extensions.rebase=!" results in the command not being suggested. We'll fix that next.

sparse: inline signature cache clearing It is a trivial one-liner. No need to have a separate function.