From: Kristofer Karlsson <krka@spotify.com>

get_reachable_subset() and tips_reachable_from_bases() answer the
same question: given a set of bases and a set of tips, which tips
are reachable from at least one base?

get_reachable_subset() was introduced in fcb2c0769d (2018-11-02)
for add_missing_tags() in remote.c. tips_reachable_from_bases()
was added in cbfe360b14 (2023-03-20) as part of the ahead-behind
series. The two were never consolidated.

With a commit-graph, tips_reachable_from_bases() can have an edge:
its DFS raises the generation floor as lower targets are found,
pruning more aggressively than the static floor in
get_reachable_subset(). Without generation numbers, some edge cases
may be slower with DFS instead of BFS since the date-ordered
prio_queue naturally stays near the top of the graph, but this
should not matter in practice -- worst case both visit the full
graph down from the bases.

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.git.1781033285419.gitgitgadget@gmail.com