Implement an automated test to detect merge commits

[mbestavros]

Enarx projects should not contain merge commits, and we'd like to test incoming pull requests to ensure they do not contain any.

The best way to go about this is probably to detect whether individual commits have multiple parents. There is no simple one-line way to do this, so we'll need to implement it from scratch (probably by iterating through PR commits and analyzing them).

[frozencemetery]

Is this actually needed? Wouldn't it be caught during normal code review anyway?

[npmccallum]

@frozencemetery Never leave to human error what can be enforced by a heartless computer!

We are enforcing lots of things as tests, including linting, documentation, etc. We want to catch all these things before a human gets involved. Passing tests should be an indication that it is ready for human review.