Change: rMWf7174057a4ec7b9c74babfe74290bb000204a581 added support for MSSQL
It changed $wgDBmwschema in includes/DefaultSettings.php
from 'mediawiki' to null, apparently in the belief that no other database engines were using this setting. But PostgreSQL does use that setting, and this change adversely affects the installation process. The user can still successfully install as long they manually specify a schema name. Leaving it as the now-default value of null or empty causes an error.
I think this part of the change should be reverted, and some compensatory changes made so that MSSQL still works with the old default value. (Alas, I don't know what those changes in the MSSQL code would need to be, and couldn't test them). Adding a new feature shouldn't get to break the existing behavior of an existing feature for its convenience.
Alternatively, we could change the PostgreSQL code so that when given a null $wgDBschema it transparently converts it to "public" (the default schema for PostgreSQL). If starting from scratch I actually would prefer using "public" rather than "mediawiki" as the default schema name, but we are not starting from scratch.