From: Kristofer Karlsson <krka@spotify.com>

get_reachable_subset() and tips_reachable_from_bases() both answer
the same reachability question but use different traversal
strategies: priority queue vs depth-first search.  Consolidate them
into tips_reachable_from_bases() with a mode parameter to select
between DFS and PQ traversal, preserving the preferred strategy for
each caller.

This works cleanly because prio_queue already supports LIFO mode
(when compare is NULL), so a single prio_queue acts as either a
stack or a heap depending on the mode.

The unified traversal pushes all unseen parents at once rather than
peeking and pushing one parent at a time.  This eliminates merge
commit revisits entirely: a 2-parent merge now requires 1 visit
instead of 3.  For DFS (LIFO) mode, the first parent is pushed
last so it ends up on top of the stack, preserving first-parent
traversal order.

Parsing is deferred to pop time for DFS since parent objects carry
valid flags without a full repo_parse_commit() call.  PQ mode
parses before push so the heap can order by generation number.

Add exhaustive reachability tests that use every commit in the
grid as a tip, protecting against subtle traversal bugs such as
wrong parent ordering or premature pruning.  The existing tests
are also extended to exercise both DFS and PQ modes.

The flag in remote.c changes from 1 (bit 0) to TMP_MARK (bit 4)
because tips_reachable_from_bases() uses SEEN (bit 0) internally.
TMP_MARK is already used for deduplication earlier in the same
function and is cleared before the reachability check.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>

Submitted-As: https://lore.kernel.org/git/pull.2144.v2.git.1781178567862.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.2144.git.1781033285419.gitgitgadget@gmail.com