Previously, only "github.com" mapped to "ssh.github.com" via ssh config
was treated as "github.com". Now, any "ssh.github.com" host is treated
as "github.com", even if it was initially aliased as something else in
the user's ssh hostname mappings.
- Per ssh_config(5), keywords and arguments may be separated by an `=`
sign as well as whitespace.
- When following the `Include` directive, skip directories that were
returned as the result of globbing.
- Respect the `Host` context when recursing into `Include`s
- Avoid having tests read from the actual filesystem.
- Avoid repeatedly looking up the home directory.
