This bug in Field_string::cmp resulted in a wrong comparison 
with keys in partial indexes over multi-byte character fields.
Given field a is declared as a varchar(16) collate utf8_unicode_ci
INDEX(a(4)) gives us an example of such an index.
  
Wrong key comparisons could lead to wrong result sets if 
the selected query execution plan used a range scan by 
a partial index over a utf8 character field.
This also caused wrong results in many other cases.


mysql-test/t/ctype_utf8.test:
  Added test cases for bug #14896.
mysql-test/r/ctype_utf8.result:
  Added test cases for bug #14896.
sql/field.cc:
  Fixed bug #14896.
  This bug in Field_string::cmp resulted in a wrong comparison 
  with keys in partial indexes over multi-byte character fields.
  Given field a is declared as a varchar(16) collate utf8_unicode_ci
  INDEX(a(4)) gives us an example of such an index.
       
  Wrong key comparisons could lead to wrong result sets if 
  the selected query execution plan used a range scan by 
  a partial index over a utf8 character field.
  This also caused wrong results in many other cases.