feat: add is_ancestor(), like git merge-base --is-ancestor

[ameyypawar]

Created by Claude Code on behalf of Amey, who reviewed it before submitting. Everything below this line is the agent's writing, not his.

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Summary

• Add gix_revision::merge_base::is_ancestor(), which tells whether a commit can be reached from another, as asked for in dedicated is-ancestor / reachability check (like `git merge-base --is-ancestor`) #2914 and listed first in Mergebase support #2795. It is built on the existing merge-base painting, as suggested in dedicated is-ancestor / reachability check (like `git merge-base --is-ancestor`) #2914.
• Add Repository::is_ancestor() and Repository::is_ancestor_with_graph(), mirroring merge_base() and merge_base_with_graph(). is_ancestor() carries a git2 alias for graph_descendant_of(), which takes the descendant first and doesn't count a commit as its own descendant.
• merge_base() is unchanged.

Commits

• feat: add merge_base::is_ancestor() to check reachability like git merge-base --is-ancestor (gix-revision)
• feat: add Repository::is_ancestor() and Repository::is_ancestor_with_graph() (gix)

Git baseline

git merge-base --is-ancestor A B succeeds if A can be reached from B, including when both are the same commit, and fails for unrelated histories. With a commit-graph, Git doesn't walk commits with a lower generation than A.

is_ancestor() does the same and ends the walk once only commits with a lower generation than A remain. It also answers false without any traversal if B is in the commit-graph but A isn't, as a commit-graph contains all ancestors of its commits, or if A's generation isn't lower than B's. Only nonzero, unsaturated generations are used.

Validation

• cargo test — gix-revision merge-base 11, gix repository::revision 5, also with GIX_TEST_FIXTURE_HASH=sha256 and with GIX_TEST_IGNORE_ARCHIVES=1
• cargo clippy -p gix-revision -p gix --all-targets, cargo fmt --check, cargo doc with -D warnings for both crates, and cargo check without the revision or merge_base features
• The new validate test takes its expectations from the existing git merge-base --all baselines: A is an ancestor of B exactly if A is their only merge-base. That rule agrees with git merge-base --is-ancestor (git 2.52.0) on all 103 single-pair baselines.
• A differential run of 2,150 commit pairs from this repository's history against git merge-base --is-ancestor (git 2.52.0) — 1,078 of them ancestors, with walks through merges and 50 pairs of identical commits — agrees on every pair without a commit-graph, with a complete one, and with one that lacks the newest 1,499 commits.
• Disabling either shortcut, the reliability check of the second one or the walk's lower bound, dropping the self-ancestor case, or checking the wrong color makes at least one new test fail. The first commit builds and passes the gix-revision tests on its own.

Not addressed here

• Mergebase support #2795 also lists merge-base --independent, which would expose remove_redundant(). Happy to follow up on that separately if it's wanted.
• Like merge_base(), Repository::is_ancestor() uses the commit-graph whenever core.commitGraph allows it, so the TODO in Mergebase support #2795 about not using commit-graphs when grafts or object replacements are in effect applies to both.
• cargo doc -p gix --no-deps with -D warnings stops on an unresolved link to Self::with_fetch_options in gix/src/clone/access.rs unless blocking-network-client is enabled. It's unrelated to this change, so it's left alone.