add a repair script for path_hash mismatching md5(path)

[butonic]

If path_hash mismatches md5(path) the file cannot be deleted. On mysql a fix would be

UPDATE oc_filecache SET path_hash = md5(path) WHERE path_hash != md5(path);

postgresql also seems to have md5(). oracle is of course weird:

select rawtohex(
    DBMS_CRYPTO.Hash (
        UTL_I18N.STRING_TO_RAW ('foo', 'AL32UTF8'),
        2)
    ) from dual;
-- output:
ACBD18DB4CC2F85CEDEF654FCCC4A4D8

mssql:

SELECT CONVERT(NVARCHAR(32),HashBytes('MD5', 'email@dot.com'),2)

sqlite does not have a md5() but we can add it as a callback in php:

$pdo = $args->getConnection()->getWrappedConnection();
$pdo->sqliteCreateFunction('md5', 'md5', 1);

see https://github.com/owncloud/core/pull/13956/files#diff-59f4e514d8f6ff0c90fd80dfafaf6f53R62

cc @PVince81 @icewind1991