commit-reach: deduplicate queue entries in paint_down_to_common

commit-reach: deduplicate queue entries in paint_down_to_common

paint_down_to_common() can enqueue the same commit multiple times
when it is reached through different parents with different flag
combinations. Add an ENQUEUED flag to track whether a commit is
currently in the priority queue, and skip it if already present.

Introduce prio_queue_put_dedup() and prio_queue_get_dedup()
wrappers that manage the ENQUEUED flag on enqueue and dequeue.

This change is performance-neutral on its own: the O(n)
queue_has_nonstale() scan still dominates the per-iteration cost.
However, the deduplication guarantee (each commit appears in the
queue at most once) is a prerequisite for the next commit, which
replaces that scan with O(1) tracking.

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