commit-graph: fix GRAPH_MIN_SIZE

The GRAPH_MIN_SIZE macro should be the smallest size of a parsable
commit-graph file. However, the minimum number of chunks was wrong.
It is possible to write a commit-graph file with zero commits, and
that violates this macro's value.

Rewrite the macro, and use extra macros to better explain the magic
constants.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -35,10 +35,11 @@
 
 #define GRAPH_LAST_EDGE 0x80000000
 
+#define GRAPH_HEADER_SIZE 8
 #define GRAPH_FANOUT_SIZE (4 * 256)
 #define GRAPH_CHUNKLOOKUP_WIDTH 12
-#define GRAPH_MIN_SIZE (5 * GRAPH_CHUNKLOOKUP_WIDTH + GRAPH_FANOUT_SIZE + \
-			GRAPH_OID_LEN + 8)
+#define GRAPH_MIN_SIZE (GRAPH_HEADER_SIZE + 4 * GRAPH_CHUNKLOOKUP_WIDTH \
+			+ GRAPH_FANOUT_SIZE + GRAPH_OID_LEN)
 
 char *get_commit_graph_filename(const char *obj_dir)
 {