Support repositories cloned into subfolder

[dorny]

If repository is checked out into subfolder of $GITHUB_WORKSPACE, our change detection via git diff-index will fail. Checkout to subfolder is not common scenario but there are valid use cases for that. One example is checkout out multiple repositories.

Solution would be to add working-directory input parameter that would allow user to specify folder where change detection (i.e. git commands) should be executed. This option won't have any impact on change detection for pull requests using Github API.