Deal with "master" -> "main" branch name changes

[gdevic]

Github moved to use the word "main" instead of "master" https://stevenmortimer.com/5-steps-to-change-github-default-branch-from-master-to-main/ There are a few hard-coded "master" strings in GitForce; make it a more flexible, per repro, option (don't assume everyone else will adopt "main".)