When no tests are given to rebaseline-cl, it will automatically rebaseline all the failing tests in the try jobs.

I find this behaviour very useful. And sometimes, I'd like to rebaseline all the failing tests in a certain directory (and stay away from the others to avoid unintentional baseline/expectation changes). Yet `... rebaseline-cl foo/` would rebaseline all tests in foo/ instead of just the failing ones.

We can add a --only-failing-tests flag to the subcommand.

CC'ing a few folks to know if this would be also useful to you.