completion: fix completion of certain aliases

Some commands need the first word to determine the actual action that is
being executed, however, the command is wrong when we use an alias, for
example 'alias.p=push', if we try to complete 'git p origin ', the
result would be wrong because __git_complete_remote_or_refspec() doesn't
know where it come from.

So let's override words[1], so the alias 'p' is override by the actual
command, 'push'.

Reported-by: Aymeric Beaumet <aymeric.beaumet@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>