pretty: add infrastructure for commit format aliases

pretty: add infrastructure for commit format aliases

Allow named commit formats to alias one another; find_commit_format() will
recursively dereference aliases when they are specified.  At this point,
there are no aliases specified and there is no way to specify an alias,
but the support is there for any which are added.

If an alias loop is detected, the function die()s.

Signed-off-by: Will Palmer <wmpalmer@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>