Refactor main committime method to reduce complexity

[etsauer]

This should resolve the final linting error as well.

@redhat-cop/mdt

[etsauer]

To test:

1. Log in to openshift (with build data present)
2. Start exporter. python exporters/committime/app.py
3. Once started, run the following and confirm you get results back: curl localhost:8080 | grep -e 'commit_timestamp{.*} [0-9]\.[0-9]\{1,\}e+[0-9]\{2\}'

[themoosman]

LGTM