commit-reach: use object flags for tips_reachable_from_bases()

commit-reach: use object flags for tips_reachable_from_bases()

tips_reachable_from_bases() walks the commit graph from a set of base
commits to find which tip commits are reachable.  The inner loop does
a linear scan over the tips array to check whether each visited commit
is a tip, making the overall cost O(C * T) where C is commits walked
and T is the number of tips.

Use the RESULT object flag to mark tip commits, replacing the linear
scan with a single flag test per visited commit.  This reduces the
per-commit tip check from O(T) to O(1) and the overall cost from
O(C * T) to O(C + T).

When multiple refs point to the same commit, the shared object gets
the flag once, so all duplicates are handled automatically.  The
early-termination advancement loop checks the flag on the sorted
commits array directly, which naturally handles duplicates since the
flag is on the shared commit object.

This also removes the index field from struct commit_and_index, since
the indirection through the original tips array is no longer needed.

This function is called by `git for-each-ref --merged` and
`git branch/tag --contains/--no-contains` via reach_filter() in
ref-filter.c.

Benchmark on a merge-heavy monorepo (2.3M commits, 10,000 refs):

  Command                           Before    After   Speedup
  for-each-ref --merged HEAD        6.57s     1.59s     4.1x
  for-each-ref --no-merged HEAD     6.67s     1.66s     4.0x
  branch --merged HEAD              0.68s     0.61s      10%
  branch --no-merged HEAD           0.65s     0.61s       8%
  tag --merged HEAD                 0.12s     0.12s       -

On linux.git with 10,000 synthetic branches at the root commit (worst
case for the DFS walk):

  Command                           Before    After   Speedup
  for-each-ref --merged HEAD        1.35s     0.35s     3.9x
  for-each-ref --no-merged HEAD     1.82s     0.31s     5.9x

The large speedup for for-each-ref is because it checks all 10,000
refs as tips, making the O(T) inner loop expensive.  The branch
subcommand only checks local branches (fewer tips), so the improvement
is smaller.

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