Field_varstring::store
      
The code that temporary saved the bitmaps of the read set and the write set so that
it can set it to all columns for debug purposes was not expecting that the
table->read_set and table->write_set can be the same. And was always saving both in 
sequence.
As a result the original value was never restored.
Fixed by saving & restoring the original value only once if the two sets are the
same (in a special set of functions).

mysql-test/r/select.result:
  Bug #37936: test case
mysql-test/t/select.test:
  Bug #37936: test case
sql/item_cmpfunc.cc:
  Bug #37936: don't save/restore twice if the read and write sets are the same
sql/opt_range.cc:
  Bug #37936: don't save/restore twice if the read and write sets are the same
sql/table.h:
  Bug #37936: Make a designated set of functions that save/restore
  both the read and the write sets in a single call.