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