Notification of string changes

Author: Jun Pataleta
MDL-64958 grade_import: Ensure correct user is being fetched
* We need to ensure that we are checking the correct user account.
Since email and idnumber are not unique fields, there's a chance that
multiple user records will match when querying for user data using
these fields. This might lead to a different user's grades being
inadvertently modified during grade import. In such a case, this
function needs to return a null userid.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=fc6d1fe71946a6f792b0862dc37f29c84e370b2e
http://github.com/moodle/moodle/commit/fc6d1fe71946a6f792b0862dc37f29c84e370b2e

+ 3.6 en [usermappingerrormultipleusersfound,core_grades]