t6600: add tests for duplicate tips in tips_reachable_from_bases()

t6600: add tests for duplicate tips in tips_reachable_from_bases()

When multiple refs point to the same commit, the reachability check
must handle them correctly.  Add three tests:

 - duplicate tips, all reachable
 - duplicate tips, none reachable
 - duplicate tips at the minimum generation (exercises the
   early-termination advancement logic)

Suggested-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>