t6099, t6600: add side-exhaustion regression tests

t6099, t6600: add side-exhaustion regression tests

Add t6099 to test the case where multiple merge-base candidates exist
and one is an ancestor of another. This exercises the side-exhaustion
optimization in paint_down_to_common together with the
remove_redundant safety net in get_merge_bases_many_0.

Add a mixed finite/INFINITY test to t6600 where one tip is outside
the commit-graph (INFINITY generation) and the other is inside.
This exercises the region transition: the walk starts in the
INFINITY region where side-exhaustion is disabled, then crosses
into the finite region where it can fire.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>