Create PR checker lane

[Neil3079]

I propose a new lane:- PRChecker

This lane will be similar to the test lane with one enhancement, it will checkout the latest version of the target branch and merge the pr into that locally before running the tests.

Some CI's allow this configured manually but not all. It is useful to ensure nothing breaks once the merge is complete before we merge on the repo however.

[KaneCheshire]

Ooh I like this idea.

[Neil3079]

The hardest part is figuring out the target branch, perhaps we could rely on a user populated ENV variable?

[zacoid55]

Might be quite hard to enforce an ENV variable, @Neil3079. Unless it would be specified in the Fastlane command via terminal?

[Neil3079]

It could default to develop as the most common case

[Neil3079]

different systems PR checkers work in different ways, if people can edit these to set an environment variable we could read it. I don't really see another way of identifying the target branch.

For instance TRAVIS CI sets the env variable TRAVIS_BRANCH for the target branch, we could listen for that in the MasterFastfile but its CI specific I think it should be up to the user to provide this info, they could do this by copying the value of this variable to one we define (perhaps TAB_TARGET_BRANCH) in the setup tasks or even in their own CI script

[zacoid55]

Could we make a shell script that could automatically check to see if any of those ENV vars exist and if not ask for a user-specified branch name?

[Neil3079]

We could but then the MasterFastfile takes on CI specific responsibilities which we have always avoided in the past. Personally I'd not against adding that script, its more complex to do on the MasterFastfile side of things and will only fix help the percentage of people who are not merging to develop and are using one of the systems we have specified.

We would then have to maintain this script and make sure it still works on future versions of these systems.