commit-graph: return 64-bit generation number

In a preparatory step for introducing corrected commit dates, let's
return timestamp_t values from commit_graph_generation(), use
timestamp_t for local variables and define GENERATION_NUMBER_INFINITY
as (2 ^ 63 - 1) instead.

We rename GENERATION_NUMBER_MAX to GENERATION_NUMBER_V1_MAX to
represent the largest topological level we can store in the commit data
chunk.

With corrected commit dates implemented, we will have two such *_MAX
variables to denote the largest offset and largest topological level
that can be stored.

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -11,8 +11,8 @@
 #include "commit-slab.h"
 
 #define COMMIT_NOT_FROM_GRAPH 0xFFFFFFFF
-#define GENERATION_NUMBER_INFINITY 0xFFFFFFFF
-#define GENERATION_NUMBER_MAX 0x3FFFFFFF
+#define GENERATION_NUMBER_INFINITY ((1ULL << 63) - 1)
+#define GENERATION_NUMBER_V1_MAX 0x3FFFFFFF
 #define GENERATION_NUMBER_ZERO 0
 
 struct commit_list {