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/vcs-svn/fast_export.c b/vcs-svn/fast_export.c
index 6c9f286..5a89db3 100644
--- a/vcs-svn/fast_export.c
+++ b/vcs-svn/fast_export.c
@@ -68,7 +68,7 @@ void fast_export_modify(const char *path, uint32_t mode, const char *dataref)
 }
 
 void fast_export_begin_note(uint32_t revision, const char *author,
-		const char *log, unsigned long timestamp, const char *note_ref)
+		const char *log, timestamp_t timestamp, const char *note_ref)
 {
 	static int firstnote = 1;
 	size_t loglen = strlen(log);
@@ -93,7 +93,7 @@ static char gitsvnline[MAX_GITSVN_LINE_LEN];
 void fast_export_begin_commit(uint32_t revision, const char *author,
 			const struct strbuf *log,
 			const char *uuid, const char *url,
-			unsigned long timestamp, const char *local_ref)
+			timestamp_t timestamp, const char *local_ref)
 {
 	static const struct strbuf empty = STRBUF_INIT;
 	if (!log)