TreeRepository's findRoots query incorrectly when using a custom primary key

[ilyasfoo]

Issue type:

[x] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[x] mysql / mariadb
[ ] oracle
[ ] postgres
[ ] cockroachdb
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[x] latest
[ ] @next
[ ] 0.x.x (or put your version here)

Related to #2361

When we use a custom primary column, the query breaks because it seems findRoots makes an assumption that the primary key = id.

An example if we use cat_id as the primary key, findRoots makes the following SQL query:

SELECT
    `treeEntity`.`cat_id` AS `treeEntity_cat_id`,
    `treeEntity`.`cat_name` AS `treeEntity_cat_name`,
    `treeEntity`.`mpath` AS `treeEntity_mpath`,
    `treeEntity`.`catParentCatId` AS `treeEntity_catParentCatId`
FROM
    `category` `treeEntity`
WHERE
    `treeEntity`.`catParentId` IS NULL '

Instead of the correct query:

SELECT
    `treeEntity`.`cat_id` AS `treeEntity_cat_id`,
    `treeEntity`.`cat_name` AS `treeEntity_cat_name`,
    `treeEntity`.`mpath` AS `treeEntity_mpath`,
    `treeEntity`.`catParentCatId` AS `treeEntity_catParentCatId`
FROM
    `category` `treeEntity`
WHERE
    `treeEntity`.`catParentCatId` IS NULL '