add --only-this-dir/-x argument to only work on a subdirectory of the repository

[toelke]

this is useful for multi-repo workflow. I am unsure about the -x, but it mirrors du -x and df -x, so it seemed appropriate?

It changes the following:

• it adds the option --only-this-dir/-x to only work on a subdirectory of the repository

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog