Problem: altering a bit field we use Field::is_equal() to check if the bit 
field is changed. Comparing the field type is not enough for bit fields.
Fix: add proper Field_bit::is_equal() that compares the field lengths as well.



mysql-test/r/type_bit.result:
  Fix for bug #28631: Bit index creation failure after alter
    - test result.
mysql-test/t/type_bit.test:
  Fix for bug #28631: Bit index creation failure after alter
    - test result.
sql/field.cc:
  Fix for bug #28631: Bit index creation failure after alter
    - Field_bit::is_equal() added.
sql/field.h:
  Fix for bug #28631: Bit index creation failure after alter
    - Field_bit::is_equal() added.