bugfix: ALTER TABLE and TIMESTAMPs around DST change time

Implement a special Copy_func function for timestamps, that copies
timestamps without converting them to MYSQL_TIME (the conversion is
lossy around DST change time).

This fixes ALTER TABLE part of main.old-mode test.

This is 10.2 version of f4f48e06

sql/field.h

@@ -699,6 +699,7 @@ class Field: public Value_source
   static void do_field_real(Copy_field *copy);
   static void do_field_string(Copy_field *copy);
   static void do_field_temporal(Copy_field *copy);
+  static void do_field_timestamp(Copy_field *copy);
   static void do_field_decimal(Copy_field *copy);
 public:
   static void *operator new(size_t size, MEM_ROOT *mem_root) throw ()
@@ -2388,6 +2389,7 @@ class Field_timestamp :public Field_temporal {
 		  TABLE_SHARE *share);
   enum_field_types type() const { return MYSQL_TYPE_TIMESTAMP;}
   enum ha_base_keytype key_type() const { return HA_KEYTYPE_ULONG_INT; }
+  Copy_func *get_copy_func(const Field *from) const;
   int  store(const char *to,uint length,CHARSET_INFO *charset);
   int  store(double nr);
   int  store(longlong nr, bool unsigned_val);

sql/field_conv.cc

@@ -417,6 +417,16 @@ void Field::do_field_decimal(Copy_field *copy)
 }
 
 
+void Field::do_field_timestamp(Copy_field *copy)
+{
+  Field_timestamp *f= static_cast<Field_timestamp*>(copy->from_field);
+  Field_timestamp *t= static_cast<Field_timestamp*>(copy->to_field);
+  ulong sec_part;
+  my_time_t ts= f->get_timestamp(&sec_part);
+  t->store_TIME(ts, sec_part);
+}
+
+
 void Field::do_field_temporal(Copy_field *copy)
 {
   MYSQL_TIME ltime;
@@ -706,6 +716,16 @@ void Copy_field::set(Field *to,Field *from,bool save)
 }
 
 
+Field::Copy_func *Field_timestamp::get_copy_func(const Field *from) const
+{
+  Field::Copy_func *copy= Field_temporal::get_copy_func(from);
+  if (copy == do_field_temporal && from->type() == MYSQL_TYPE_TIMESTAMP)
+    return do_field_timestamp;
+  else
+    return copy;
+}
+
+
 Field::Copy_func *Field_temporal::get_copy_func(const Field *from) const
 {
   /* If types are not 100 % identical then convert trough get_date() */