errors: create "similarity hint" for UnknownIdentifier eagerly in constructor No code wanted to do anything but to produce a hint from it anyway, so we might as well just store the hint in the exception (which already extended `Hint`). That way we can easily convert it to a `ConfigException` when it's parsing of configuration that fails. I was wondering if the purpose of lazily creating the string was so we don't create it in cases where it won't get printed anyway. However, I couldn't find any places where that could happen. If we do find such places, we could instead revert to making it lazy but add a function on `UnknownIdentifier` for creating the hint string. I dropped the comment saying "make sure to check fileset first, as revset can invoke fileset", which was added in 4e240d6ab898 (dispatch: offer near-edit-distance suggestions for {file,rev}set functions, 2015-01-26). I couldn't figure out what it meant. The author of that patch also did not remember the reason for it. Perhaps changes that have happened since then made it so it no longer matters. Differential Revision: https://phab.mercurial-scm.org/D9346

errors: move similarity_hint() to error module I want to be able to reuse it from `UnknownIdentifier`'s constructor. Moving it results in a new import of `difflib` in the `error` module. There was a comment at the top of `error.py` saying "Do not import anything but pycompat here, please", which was added (except for the "pycompat" bit) in 08cabecfa8a8 (errors: move revlog errors, 2009-01-11). I don't know the reason for the comment. I'm guessing the point was to not make the module depend on other Mercurial modules. If that was it, then importing `difflib` should be fine. Sorry about the churn (I moved this code from the `dispatch` module to the `scmutil` module very recently). Differential Revision: https://phab.mercurial-scm.org/D9345