revision: allow --ancestry-path to take an argument

revision: allow --ancestry-path to take an argument

We have long allowed users to run e.g.
    git log --ancestry-path master..seen
which shows all commits which satisfy all three of these criteria:
  * are an ancestor of seen
  * are not an ancestor of master
  * have master as an ancestor

This commit allows another variant:
    git log --ancestry-path=$TOPIC master..seen
which shows all commits which satisfy all of these criteria:
  * are an ancestor of seen
  * are not an ancestor of master
  * have $TOPIC in their ancestry-path
that last bullet can be defined as commits meeting any of these
criteria:
    * are an ancestor of $TOPIC
    * have $TOPIC as an ancestor
    * are $TOPIC

This also allows multiple --ancestry-path arguments, which can be
used to find commits with any of the given topics in their ancestry
path.

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>