feat: enable relative path ssh URL

[JyJyJcr]

Enable relative path in SCP style URL if the host is not github.com. This allows to specify the ssh repository by shorter URL, like http ones.

[Songmu]

Thank you. We are having trouble with this URL style, especially since there is no clear specification. Maybe this change won't be a big problem, so I would like to merge it. Please fix the typo in the documentation beforehand.

[JyJyJcr]

Typo fixed. Thank you for reviewing!

[Songmu]

Thank you for your contribution.
Considering the large implementation differences in SSH and git servers and the impact on the existing environment, I will revert this change.
I thought I should treat only GitHub especially, but Gitlab has the same problem, and in some cases, GitHub Enterprise is using GitHub on a custom domain.

[JyJyJcr]

I understood the problem. Thank you for your notice!