[Feature] Introduce entry field for merge flag

Hey,

I managed to extend your work and would like to give something back.
I introduced another field in the job configuration for a merge flag like --no-ff

What's missing is a validation of the content, which I left for you to add.
Since I could not compile with Tests on, I could provide automatic Tests, but it works as it should.
This addition works together with a change of the git-client plugin.

Method: Taking input string, pass it to the call of git-client and store it in an evironment variable.
The git-client-plugin gets it form there.

Thanks and fare well