timestamp_t: a new data type for timestamps

Git's source code assumes that unsigned long is at least as precise as
time_t. Which is incorrect, and causes a lot of problems, in particular
where unsigned long is only 32-bit (notably on Windows, even in 64-bit
versions).

So let's just use a more appropriate data type instead. In preparation
for this, we introduce the new `timestamp_t` data type.

By necessity, this is a very, very large patch, as it has to replace all
timestamps' data type in one go.

As we will use a data type that is not necessarily identical to `time_t`,
we need to be very careful to use `time_t` whenever we interact with the
system functions, and `timestamp_t` everywhere else.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/archive-tar.c b/archive-tar.c
index 380e3ae..319a5b1 100644
--- a/archive-tar.c
+++ b/archive-tar.c
@@ -27,9 +27,12 @@ static int write_tar_filter_archive(const struct archiver *ar,
  */
 #if ULONG_MAX == 0xFFFFFFFF
 #define USTAR_MAX_SIZE ULONG_MAX
-#define USTAR_MAX_MTIME ULONG_MAX
 #else
-#define USTAR_MAX_SIZE 077777777777UL
+#define USTAR_MAX_SIZE 077777777777ULL
+#endif
+#if TIME_MAX == 0xFFFFFFFF
+#define USTAR_MAX_MTIME TIME_MAX
+#else
 #define USTAR_MAX_MTIME 077777777777UL
 #endif