Don't use full-path when passing it to Credo

[paulswartz]

Credo seems to check the whole repository when a full path is given to it. This PR changes that to only pass the relative path.

It additionally relaxes the check for line numbers: this allows it to catch changes like "pipe chain should start with a raw value" which tend to be triggered by different line from what's in the patch.

[carakan]

Thanks @paulswartz for this PR's, I'll take a look your changes shortly.

[carakan]

@paulswartz I reviewed your changes, that seems pretty good, but I see a lot of new messages per commit. I pushed a new branch develop in that tagging version 0.0.5 and testing and comparing with 0.0.4 the new version adds new messages, I need to compare if your new changes makes a better handling of changes than older version or adds unrelated messages.

[paulswartz]

It definitely adds more messages: what it tries to do is find errors on adjacent lines. Consider this diff:

   defp do_build_matcher({key_str, value_str}, acc) do
     values = integer_values(value_str)
+    |> Enum.map(& &1)
     matchers_for_values(acc, key_str, values)
   end

The only line in the diff is fine. However, adding that line causes a Credo issue on the previous line (pipe chain should start with a raw value). My branch tries to find those nearby issues and include them as messages, since they are related to the commit.