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>

archive.h

@@ -9,7 +9,7 @@ struct archiver_args {
 	struct tree *tree;
 	const unsigned char *commit_sha1;
 	const struct commit *commit;
-	time_t time;
+	timestamp_t time;
 	struct pathspec pathspec;
 	unsigned int verbose : 1;
 	unsigned int worktree_attributes : 1;