Travis puts the commit message in an environment variable, so if it
contains the string `_flag` this would match TRAVIS_COMMIT_MESSAGE.
That happened in ca91c201c3, so the
tests failed.
We simply tighten the regex a little more, and make a commit message
that doesn't include the string.
This is a nice test (ha!) for how this works and what littlecheck can
do for us.
1. Input is now the actual file, not "Standard Input" anymore. So
any errors mentioning that now include the filename.
2. Regex are really nice for filenames, but especially for line
numbers
3. It's much nicer to have the output where it's created, instead of
needing to follow three files at the same time.
