Cloning/Importing a project using SSH URLs is causing a server error 500

Summary

When using an [unauthenticated] ssh-URL (the default option in e.g. GitHub) to import a new project I'm getting a 500 server error.

Note that I'm not using the git:// scheme on the URL (prepending it actually causes: https://gitlab.com/gitlab-org/gitlab-ee/issues/917)

Steps to reproduce

New Project
Select "Import project from" > "Repo by URL"
Paste the URL. e.g. git@github.com:gitlabhq/gitlabhq.git (implicitly a git://)

or cURL:

curl 'https://gitlab.com/projects' \
    -H 'Host: gitlab.com' \
    -H 'Content-Type: application/x-www-form-urlencoded' \
    [...]
    --data 'utf8=✓&    
        authenticity_token=<auth_token>&
        project[template_name]=&
        project[import_url]=git@github.com:gitlabhq/gitlabhq.git&
        project[mirror_user_id]=<mirror_user_id>&
        project[namespace_id]=<namespace_id>&
        project[path]=gitlabhq&
        project[description]=&
        project[visibility_level]=0'

Example Project

N/A (projects cannot actually be created with this)

What is the current bug behavior?

Request is replied with a 500 server error.

What is the expected correct behavior?

Project is imported.

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's very hard to read otherwise.)

Output of checks

This bug happens on GitLab.com