graph: handle not finding a merge base gracefully

git_merge_base() returns GIT_ENOTFOUND when it cannot find a merge
base. graph_desdendant_of() returns a boolean value (barring any
errors), so it needs to catch the NOTFOUND return value and convert it
into false, as not merge base means it cannot be a descendant.

src/graph.c

@@ -180,7 +180,12 @@ int git_graph_descendant_of(git_repository *repo, const git_oid *commit, const g
 	if (git_oid_equal(commit, ancestor))
 		return 0;
 
-	if ((error = git_merge_base(&merge_base, repo, commit, ancestor)) < 0)
+	error = git_merge_base(&merge_base, repo, commit, ancestor);
+	/* No merge-base found, it's not a descendant */
+	if (error == GIT_ENOTFOUND)
+		return 0;
+
+	if (error < 0)
 		return error;
 
 	return git_oid_equal(&merge_base, ancestor);