Allow choice of branch when configuring the signing with match in remote repos

[GMinucci]

Background

Fastlane Match allows you to store multiple certificates and provisionings in the same repo in separate branches. By default it will always use the one in master branch but you can choose any branch. This configuration is not available through variants signing options.

Solution

To add one optional parameter to the signing configurations in variants.yml that allows you to choose a specific branch to sign.