Add robustness to repo source not having a slash

[maelle]

In https://github.com/r-lib/pkgdown/blob/6a412e3a9d2d12d43e6b24eb25b97633ec350cda/R/repo.R#L25 if one has e.g.

repo:
  url:
    home: https://gitlab.com/chuxinyuan/ecce
    source: https://gitlab.com/chuxinyuan/ecce/-/tree/master

without "/" the links to code source are wrong as they are, well, missing a slash.

Sort of related to #2015

[chuxinyuan]

Sorry, I was unable to access github for a long time. This issue seems to have received little attention. If necessary, I would like to cooperate with you to complete the development and testing work.

[hadley]

Since we already have a fs dependency, I think we can just switch to fs::path() here.

[hadley]

Ah that doesn't work because it strips the double // from http://