Does not update max_migration.txt when a merge migration is created.

[tolomea]

Obviously I had to bypass the system checks to even get this to run.
We're not too concerned about having a linear history but would really like migration conflicts to be git conflicts.
I plan to encourage a move toward linear anyway by encouraging use of rebase, but the transition onto this tool would be easier if my team mates didn't have to go cold turkey on merges.
Incidentally it happens because in the underlying code --merge does not go through write_migration_files it calls MigrationWriter directly.