Merge branch 'jc/alias-completion'

The command line completion script (in contrib/) can be told to
complete aliases by including ": git <cmd> ;" in the alias to tell
it that the alias should be completed similar to how "git <cmd>" is
completed.  The parsing code for the alias as been loosened to
allow ';' without an extra space before it.

* jc/alias-completion:
  completion: loosen and document the requirement around completing alias