Expand shell variables in inputs

[tbroadley]

Closes #123.

This PR fixes the issue in #123 by going back to constructing hub pull-request arguments in a string variable, rather than in an array. entrypoint.sh also goes back to running the hub pull-request command using sh -c.

I also considered using eval echo to evaluate $INPUT_PR_TITLE . But I'm given to understand that we should avoid eval in shell scripts. Although I think sh -c is equivalent, so maybe it isn't important to avoid using eval here.

[wei]

Thank you @tbroadley we'll do some testing and get this merged! For now please use repo-sync/pull-request@v2.9

[tbroadley]

Hey @wei , I wanted to check if you've had a chance to test this yet.

[JohnYoungers]

@wei bump: while the issue is superficial, it would be nice if we could get our PRs back to displaying accurate details