parse_timestamp(): specify explicitly where we parse timestamps

Currently, Git's source code represents all timestamps as `unsigned
long`. In preparation for using a more appropriate data type, let's
introduce a symbol `parse_timestamp` (currently being defined to
`strtoul`) where appropriate, so that we can later easily switch to,
say, use `strtoull()` instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -319,6 +319,8 @@ extern char *gitdirname(char *);
 #define PRIo32 "o"
 #endif
 
+#define parse_timestamp strtoul
+
 #ifndef PATH_SEP
 #define PATH_SEP ':'
 #endif