Can't push via pr-train when no local master is checked out

[upsuper]

Because usually I branch from green rather than master (as recommended), I don't generally checkout master. But when there is no local branch named "master", pr-train would throw this error when pushing:

fatal: malformed object name master

❌  An error occured. Was there a conflict perhaps?
error Error: fatal: malformed object name master

    at toError (/usr/local/lib/node_modules/git-pr-train/node_modules/simple-git/promise.js:90:14)
    at /usr/local/lib/node_modules/git-pr-train/node_modules/simple-git/promise.js:61:36
    at Git.<anonymous> (/usr/local/lib/node_modules/git-pr-train/node_modules/simple-git/src/git.js:632:18)
    at Function.Git.fail (/usr/local/lib/node_modules/git-pr-train/node_modules/simple-git/src/git.js:1474:18)
    at fail (/usr/local/lib/node_modules/git-pr-train/node_modules/simple-git/src/git.js:1432:20)
    at /usr/local/lib/node_modules/git-pr-train/node_modules/simple-git/src/git.js:1441:16
    at process._tickCallback (internal/process/next_tick.js:68:7)

maybe it should track origin/master instead?