feat(git): treat altssh.gitlab.com as official GitLab.com SSH alias

Mario Celirequested to merge
fix-altssh-gitlab-com-translation into main

What does this MR do?

Fixes remote resolution for users whose ~/.ssh/config uses GitLab.com's documented alternative SSH port endpoint:

Host gitlab.com
  Hostname altssh.gitlab.com
  Port 443

With this configuration, commands like glab mr create failed with:

none of the git remotes configured for this repository point to a known GitLab host.
Configured remotes: altssh.gitlab.com

The SSH config translator (internal/git/ssh_config.go) rewrote gitlab.com remotes to altssh.gitlab.com, which is not a known/authenticated host. A special case already existed for ssh.gitlab.com; this MR extends it to cover both official GitLab.com SSH aliases.

How to test

  1. Add the altssh block above to ~/.ssh/config.
  2. In a repository with an SSH remote for gitlab.com, run glab mr create (or any command that resolves remotes).
  3. Without this change it fails with the unknown-host error; with it, remotes resolve to gitlab.com.

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