Remove the IScriptRepository as a dependency of Database.  Pass it as a method argument instead.
Add nuget to the solution.
Use a testable wrapper around the System.IO types.
Create a contract for running external processes.  To be used by the DvcsScriptRepositories.
Update GitScriptRepository to use IExternalProcess.
Update HgScriptRepository to use IExternalProcess.
Extract a mockable interface for the Database type.