Don't manually replace whitespace during render

For historical reasons Gitea manually alters the urlPrefix and replaces
a whitespace with a +. This Works for URLs, but we're also passing
urlPrefix to git calls and adding the + is breaking the tree path.

Goldmark will automatically convert a white space to the proper %20, so
we should leave the string as is which lets us pass it to git unmodified
and then let Goldmark fix it.

Also fixed separate bug in URLJoin I noticed while testing where it will
silently discard sections of a path that have # in them (possibly
others). We should just escape it first.

Fixes 10156

modules/markup/markdown/goldmark.go

@@ -52,7 +52,6 @@ func (g *GiteaASTTransformer) Transform(node *ast.Document, reader text.Reader,
 
 				lnk := string(link)
 				lnk = giteautil.URLJoin(prefix, lnk)
-				lnk = strings.Replace(lnk, " ", "+", -1)
 				link = []byte(lnk)
 			}
 			v.Destination = link