Hey folks! v3 is here!!!
Backstory: People had a bunch of issues with v2, particularly the problem of accessing private repos. It wasn't clear what the problems were, so I decided to rewrite the action and break each section into its own script. The private repo problem was solved and described in the new wiki pages. (The issue was fixed in v2.4 also.)
Please note that I renamed many variables in v3 to make their function more clear.
New - TEST MODE
Test mode runs a separate set of scripts (instead of the normal action) and verifies that your setup is correct. It checks if your input branches exist, if your upstream repo exists, if you have access to the upstream repo, and if your git config is setting and resetting properly. I guarantee that if you use test mode you'll spot problems with repo access before you run the action for real.
I also made a wiki with all of the tricky setup details, so if you're having trouble please check that out.