This option allows to use the first line of commit message as PR
title. It requires that ORIGINAL_MESSAGE is on and working (meaning the
file(s) were changed and the action was triggered by pushing a single
commit)

Fixes #103

Demo:

1. First commit sets the PR title correctly, but when a second commit is added to the PR it reverts the title: https://github.com/Nef10/test-sync/pull/81
2. Title not set because useOriginalCommitMessage is not true (as the change is not triggered by a commit): https://github.com/Nef10/test-sync/pull/82
3. With COMMIT_AS_PR_TITLE off: https://github.com/Nef10/test-sync/pull/83