Restore "arc alias" shell aliases

Summary:
See https://discourse.phabricator-community.org/t/exception-when-trying-to-run-an-arc-alias-undefined-method-arcanistalias-getshellcommand/3731/.

Shell aliases got lost in the shuffle of porting "arc alias"; restore them.

Test Plan: Bound arc ls to ls -alh, ran arc ls.

Differential Revision: https://secure.phabricator.com/D21084