* rmurl: When youtube-dl was used for an url, it no longer needs to be
prefixed with "yt:" in order to be removed.
* rmurl: If an url is both used by the web and also claimed by another
special remote, fix a bug that caused the url to to not be removed.
The youtube-dl change is a consequence of how the bug fix is implemented.
But I also think it's the right thing to do. Consider that, before,
git-annex addurl $url followed by git-annex rmurl $url would not remove the
url in the case where youtube-dl was used. That was surprising behavior.
In the unlikely case where a special remote claims an url, and it's been
added using OtherDownloader, but it was also added already as a web url,
it seems better for rmurl to remove both than to arbitrarily remove only one.
And in the case the bug report was filed for, when an url was added as a
web url, but a special remote now claims it, that should not prevent rmurl
removing the web url.
Calling setUrlMissing lets other callers of it behave differently.
Probably the calls to it in eg, Remote.External and Remote.BitTorrent are
fine, since they don't mangle the url and just remove what was provided,
and the OtherDownloader form of a bittorrent url, respectively.
I suspect unregisterurl needs to have a similar change made to rmurl, for
similar reasons.
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