rev-list: Demonstrate breakage with --ancestry-path --all

rev-list: Demonstrate breakage with --ancestry-path --all

The option added by commit ebdc94f3 (revision: --ancestry-path,
2010-04-20) does not work properly in combination with --all, at least
in the case of a criss-cross merge:

    b---bc
   / \ /
  a   X
   \ / \
    c---cb

There are no descendants of 'cb' in the history.  The command

  git rev-list --ancestry-path cb..bc

correctly reports no commits.  However, the command

  git rev-list --ancestry-path --all ^cb

reports 'bc'.  Add a test case to t6019-rev-list-ancestry-path
demonstrating this breakage.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>