title: error when using multiple EF migrations / configurations (__MigrationHistory is recreated everytime)
body: <p>i have 2 different and indipendent dbcontexts and i want to create the db schema (using npgsql/posgresql) with code first migration.
  i put in 2 different folder the configurations (with AutomaticMigrationsEnabled set to false) and the 2 initial migration files (once for every dbcontext) using the Add-Migration command line tool.</p>
  
  <p>then i use the command line Update-Database with the first configuration and it works, the db is correctly created with the tables related to the first context.
  But when i try to use Update-Database with the other configuration i get an exception because it is trying to create the table __MigrationHistory that already exists.</p>
  
  <p>what's wrong in my code?</p>