help: also hide options marked EXPERIMENTAL Similar to DEPRECATED, add a way to indicate that options are EXPERIMENTAL.

rebase: don't forward "source" argument to rebase (issue4633) `hg pull` takes an optional "source" argument to define the path/url to pull from. Under some circumstances, this option could get proxied to rebase and interpretted as the --source argument to rebase, leading to unexpected behavior. In my local environment, "source" always appears in "opts" in pullrebase. However, when attempting to write a test, I couldn't reproduce this. Instead, the source is being captured as a positional argument in "args." I suspect an interaction between **kwargs and an extension is to blame for the differences in behavior. This is why no test has been written. I have tested behavior locally and the patch has the intended side-effect of making `hg pull --rebase` work again.