When the publicurl has been set to an url that does not end with a slash,
we need to add one in between it and the rest of the url.
As far as I can see, git-annex does not default to such publicurls; it's
careful to end them with slashes. But this was observed in the wild, and
there may be documentation that doesn't include the slash. And it's an easy
mistake to make in any case.
This commit was sponsored by Eric Drechsel on Patreon.
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