Merge branch 'js/larger-timestamps'

Some platforms have ulong that is smaller than time_t, and our
historical use of ulong for timestamp would mean they cannot
represent some timestamp that the platform allows.  Invent a
separate and dedicated timestamp_t (so that we can distingiuish
timestamps and a vanilla ulongs, which along is already a good
move), and then declare uintmax_t is the type to be used as the
timestamp_t.

* js/larger-timestamps:
  archive-tar: fix a sparse 'constant too large' warning
  use uintmax_t for timestamps
  date.c: abort if the system time cannot handle one of our timestamps
  timestamp_t: a new data type for timestamps
  PRItime: introduce a new "printf format" for timestamps
  parse_timestamp(): specify explicitly where we parse timestamps
  t0006 & t5000: skip "far in the future" test when time_t is too limited
  t0006 & t5000: prepare for 64-bit timestamps
  ref-filter: avoid using `unsigned long` for catch-all data type

git-compat-util.h

@@ -319,6 +319,11 @@ extern char *gitdirname(char *);
 #define PRIo32 "o"
 #endif
 
+typedef uintmax_t timestamp_t;
+#define PRItime PRIuMAX
+#define parse_timestamp strtoumax
+#define TIME_MAX UINTMAX_MAX
+
 #ifndef PATH_SEP
 #define PATH_SEP ':'
 #endif