Added warning message for todo.sh archive

[chrysle]

I have added a warning message which todo.sh archive will print out if it does not find done tasks in the todo file.
Before, it would even then print TODO: ./todo.txt archived.

Before submitting a pull request, please make sure the following is done:

• Fork the repository and create your branch from master.
• If you've added code that should be tested, add tests!
• Ensure the test suite passes.
• Format your code with ShellCheck.
• Include a human-readable description of what the pull request is trying to accomplish.
• Steps for the reviewer(s) on how they can manually QA the changes.
• Have a fixes #XX reference to the issue that this pull request fixes.

[inkarkat]

I like the idea; it's nice to give precise messages about what has been done (or not); especially when doing so does not require extra work - the grep command already tells us whether there are done tasks. However:

After your change, archiving becomes non-functional if TODOTXT_VERBOSE=0; line 1145 just echos the done tasks if verbose is on; I think you need to use the following line's grep to check for done tasks. The tests (make test) are also failing - please add a test for your warning message to tests/t1900-archive.sh.

[inkarkat]

Thanks for adding tests! (The test failures can be ignored; there's a problem with MacOS platforms and apparently now also with Ubuntu 18.04.) I think we're almost there now.

[chrysle]

Should I squash my commits?

[inkarkat]

You can squash if you want to, but there's no need. The change is looking fine now, thanks a lot!

It'll probably still take some time (at least until January) until this gets merged, we currently have a bit of a backlog of PRs, but @karbassi has a New Year's resolution of devoting some time to this project, and we'll very likely also do a fresh release then ;-)