Urgh I made some wrong amends and I'm too tired to separate the commits out this time

ancestor: Split out of reachability
mergebase: Add merge base routines
internal/commitquery: Add commit query context engine thingy
internal/peel: Shared tag peeling
errors: Shared object query errors
internal/testgit: Add rooted repo helpers; remove raw path access
objectstore/memory: Add in-memory object store
objectid: Add Compare helper

1 files changed, 30 insertions, 0 deletions

diff --git a/internal/commitquery/ancestor.go b/internal/commitquery/ancestor.go
new file mode 100644
index 00000000..78149c6a
--- /dev/null
+++ b/internal/commitquery/ancestor.go
@@ -0,0 +1,30 @@
+package commitquery
+
+// IsAncestor reports whether ancestor is reachable from descendant through
+// commit parent edges.
+func IsAncestor(ctx *Context, ancestor, descendant NodeIndex) (bool, error) {
+	if ancestor == descendant {
+		return true, nil
+	}
+
+	ancestorGeneration := ctx.EffectiveGeneration(ancestor)
+	descendantGeneration := ctx.EffectiveGeneration(descendant)
+
+	if ancestorGeneration != generationInfinity &&
+		descendantGeneration != generationInfinity &&
+		ancestorGeneration > descendantGeneration {
+		return false, nil
+	}
+
+	minGeneration := uint64(0)
+	if ancestorGeneration != generationInfinity {
+		minGeneration = ancestorGeneration
+	}
+
+	_, err := paintDownToCommon(ctx, ancestor, []NodeIndex{descendant}, minGeneration)
+	if err != nil {
+		return false, err
+	}
+
+	return ctx.HasAnyMarks(ancestor, markRight), nil
+}