git alias: try alias last.

git alias: try alias last.

This disables alias "foo" from being used for git-foo, and when
we do use alias we check the built-in and then existing command
names first and then alias as the fallback.  This avoids the
problem of common commands used in scripts getting clobbered by
user specific aliases.

Signed-off-by: Junio C Hamano <junkio@cox.net>