When installing with a completely fresh and untouched clone of the master branch, the following two variables were written to config_inc.php like this:

$db_table_prefix = '';
$db_table_suffix = '';

i.e. without the 'g_' that they have in config_defaults_inc.php

When I added the variable name prefix, all worked as it should:

$g_db_table_prefix = '';
$g_db_table_suffix = '';