checkout bugfix: use stat.mtime instead of stat.ctime in two places

Commit e1afca4fd "write_index(): update index_state->timestamp after
flushing to disk" on 2009-02-23 used stat.ctime to record the
timestamp of the index-file.  This is wrong, so fix this and use the
correct stat.mtime timestamp instead.

Commit 110c46a909 "Not all systems use st_[cm]tim field for ns
resolution file timestamp" on 2009-03-08, has a similar bug for the
builtin-fetch-pack.c file.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-fetch-pack.c

@@ -807,7 +807,7 @@ struct ref *fetch_pack(struct fetch_pack_args *my_args,
 				die("shallow file was removed during fetch");
 		} else if (st.st_mtime != mtime.sec
 #ifdef USE_NSEC
-				|| ST_CTIME_NSEC(st) != mtime.nsec
+				|| ST_MTIME_NSEC(st) != mtime.nsec
 #endif
 			  )
 			die("shallow file was changed during fetch");

read-cache.c

@@ -1563,8 +1563,8 @@ int write_index(struct index_state *istate, int newfd)
 
 	if (ce_flush(&c, newfd) || fstat(newfd, &st))
 		return -1;
-	istate->timestamp.sec = (unsigned int)st.st_ctime;
-	istate->timestamp.nsec = ST_CTIME_NSEC(st);
+	istate->timestamp.sec = (unsigned int)st.st_mtime;
+	istate->timestamp.nsec = ST_MTIME_NSEC(st);
 	return 0;
 }