commit-graph.c: extract verify_one_commit_graph()

When the `verify_commit_graph()` function was extended to support
commit-graph chains via 3da4b60 (commit-graph: verify chains with
--shallow mode, 2019-06-18), it did so by recursively calling itself on
each layer of the commit-graph chain.

In practice this poses no issues, since commit-graph chains do not loop,
and there are few enough of them that adding additional frames to the
stack is not a problem.

A future commit will consolidate the progress output from `git
commit-graph verify` when verifying chained commit-graphs to print a
single line instead of one progress meter per commit-graph layer.
Prepare for this by extracting a routine to verify a single layer of a
commit-graph.

Note that `verify_commit_graph()` is still recursive after this patch,
but this will change in the subsequent patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ttaylorr

commit-graph.c

@@ -2543,18 +2543,14 @@ static int commit_graph_checksum_valid(struct commit_graph *g)
 	return hashfile_checksum_valid(g->data, g->data_len);
 }
 
-int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
+static int verify_one_commit_graph(struct repository *r,
+				   struct commit_graph *g,
+				   int flags)
 {
 	uint32_t i, cur_fanout_pos = 0;
 	struct object_id prev_oid, cur_oid;
 	int generation_zero = 0;
 	struct progress *progress = NULL;
-	int local_error = 0;
-
-	if (!g) {
-		graph_report("no commit-graph file loaded");
-		return 1;
-	}
 
 	verify_commit_graph_error = verify_commit_graph_lite(g);
 	if (verify_commit_graph_error)
@@ -2700,7 +2696,19 @@ int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
 	}
 	stop_progress(&progress);
 
-	local_error = verify_commit_graph_error;
+	return verify_commit_graph_error;
+}
+
+int verify_commit_graph(struct repository *r, struct commit_graph *g, int flags)
+{
+	int local_error = 0;
+
+	if (!g) {
+		graph_report("no commit-graph file loaded");
+		return 1;
+	}
+
+	local_error = verify_one_commit_graph(r, g, flags);
 
 	if (!(flags & COMMIT_GRAPH_VERIFY_SHALLOW) && g->base_graph)
 		local_error |= verify_commit_graph(r, g->base_graph, flags);