Provide a more helpful message if a user connects via raw SSH
Summary: We currently print a fairly vague, technical message which is ambiguous about indicating success or an error if you aren't familiar with SSH.
Test Plan:
$ ssh -T dweller@localhost phabricator-ssh-exec: Welcome to Phabricator. You are logged in as epriestley. You haven't specified a command to run. This means you're requesting an interactive shell, but Phabricator does not provide an interactive shell over SSH. Usually, you should run a command like `git clone` or `hg push` rather than connecting directly with SSH. Supported commands are: conduit, git-receive-pack, git-upload-pack, hg, svnserve.
Reviewers: btrahan, dctrwatson
Reviewed By: dctrwatson
CC: aran
Differential Revision: https://secure.phabricator.com/D7854