merge: add options to warn or ignore on colliding unknown files A 'colliding unknown file' is a file that meets all of the following conditions: - is untracked or ignored on disk - is present in the changeset being merged or updated to - has different contents Previously, we would always abort whenever we saw such files. With this config option we can choose to warn and back the unknown files up instead, or even forgo the warning entirely and silently back the unknown files up. Common use cases for this configuration include a large scale transition of formerly ignored unknown files to tracked files. In some cases the files can be given new names, but in other cases, external "convention over configuration" constraints have determined that the file must retain the same name as before.