parse_github_repo does not allow trailing slash in subdirectory

[richfitz]

A trailing slash in a subdirectory specification causes an error in parse_github_repo

> remotes::parse_github_repo_spec("foo/bar/baz/")
Error in remotes::parse_github_repo_spec("foo/bar/baz/") : 
  Invalid git repo: foo/bar/baz/
> remotes::parse_github_repo_spec("foo/bar/baz")
$username
[1] "foo"

$repo
[1] "bar"

$subdir
[1] "baz"

I think that the these should both point at the same place

[richfitz]

Thanks 🎉