Documentation: add a SEE ALSO section for merge-base

Documentation: add a SEE ALSO section for merge-base

For example, a person reading the merge-base man page might wonder
about the fastest way to check if one commit is an ancestor of
another (which would require rev-list).

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