fix: keep url scheme when deploy_to_branch() uses subdir

[taekop]

deploy_to_branch() appends subdir to the site url with fs::path(), which collapses the // after the scheme. A url of https://example.org/demo with subdir = "v1" ended up as https:/example.org/demo/v1, and that value ended up in the built site's pkgdown.yml.

The two parts are now joined with a single /. Only URLs that have a scheme change: for the 56 url/subdir combinations I compared before and after, the old and new values were identical except that https:/ became https:// (schemeless and local-path values are unchanged). Trailing slashes on url and leading/trailing slashes on subdir are still normalised as before.

I did not change the behaviour of appending subdir to url at all, since the issue also questions that and it looks like your call.

The new test runs deploy_to_branch(subdir = "v1") against a local bare remote and checks the URLs written to pkgdown.yml on the gh-pages branch. It fails without the fix and passes with it. On my machine the rest of the suite gives the same 12 failures on main and on this branch (the quarto article tests, which need the quarto package, plus the build_article error snapshot, the tutorials test and the figure test).

Part of #2816