Pass-through to user shell support when SSH connection is not from a git client

[neechbear]

Given the way in which the gitcache-ssh is invoked as a ForceCommand from the OpenSSH server, it would be useful for the wrapper to detect when it is not being called by a client git command line, and simply pass-through to the users original default shell (justexec` and replace itself).

This should, of course, be optionally configurable functionality, as there will be use cases where the Git case administrator will not wish to grant shell access.

If this functionality is added, it should be administratively disabled by default.