match: catch attempts to create case-insenstive exact matchers
Exact matchers are only created internally (as opposed to from user
input) based on a set of files that the caller collected before, so
they should always match the list exactly (i.e. case-sensitively).
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.