When fetch.writeCommitGraph is enabled, the commit-graph is currently
rebuilt from all reachable refs after every fetch. This is unnecessarily
expensive on repositories with many refs, since add_ref_to_set() validates
each ref against the odb.

This series optimizes the commit-graph write by using only the newly updated
refs as seeds instead of scanning all refs. It introduces a three-mode enum
(REACHABLE / TIPS / SKIP) to make the policy explicit:

 * No-op fetch: skip the commit-graph write entirely
 * Updated refs + existing graph: write incrementally from updated tips only
 * No existing graph or multi-remote fetch: fall back to full reachable scan

Patch 1 adds a commit-info subcommand to test-tool read-graph for verifying
graph contents in tests.

Patch 2 implements the optimization in builtin/fetch.c with four tests
covering the incremental, unrelated-commit, no-op, and fallback cases.

Kristofer Karlsson (2):
  test-tool read-graph: add commit-info subcommand
  fetch: write commit-graph using updated refs only

 builtin/fetch.c            | 65 ++++++++++++++++++++++++++++++++------
 commit-graph.c             |  2 +-
 commit-graph.h             |  1 +
 t/helper/test-read-graph.c | 23 +++++++++++++-
 t/t5510-fetch.sh           | 58 ++++++++++++++++++++++++++++++++++
 5 files changed, 138 insertions(+), 11 deletions(-)

base-commit: 0f8e75abebff0877cae681a3d5ff31ac47f54220

Submitted-As: https://lore.kernel.org/git/pull.2239.git.1790930019.gitgitgadget@gmail.com