gh-138535: Optimize fill_time for typical timestamps (#138537)

While file timestamps can be anything the file system can store, most
lie between the recent past and the near future.  Optimize fill_time()
for typical timestamps in three ways:

- When possible, convert to nanoseconds with C arithmetic.
- When using C arithmetic and the seconds member is not required (for
  st_birthtime), avoid creating a long object.
- When using C arithmetic, reorder the code to avoid the null checks
  implied in Py_XDECREF().

Co-authored-by: Victor Stinner <vstinner@python.org>

Lib/test/test_os.py

@@ -1064,9 +1064,15 @@ def test_large_time(self):
         if self.get_file_system(self.dirname) != "NTFS":
             self.skipTest("requires NTFS")
 
-        large = 5000000000   # some day in 2128
-        os.utime(self.fname, (large, large))
-        self.assertEqual(os.stat(self.fname).st_mtime, large)
+        times = (
+            5000000000,  # some day in 2128
+            # boundaries of the fast path cutoff in posixmodule.c:fill_time
+            -9223372037, -9223372036, 9223372035, 9223372036,
+        )
+        for large in times:
+            with self.subTest(large=large):
+                os.utime(self.fname, (large, large))
+                self.assertEqual(os.stat(self.fname).st_mtime, large)
 
     def test_utime_invalid_arguments(self):
         # seconds and nanoseconds parameters are mutually exclusive