Fix large push data getting truncated

This fixes random 500 when hooked diff is too large.
Closes #258

Changing huge amount of files or sending diff over multiple commits might result in a payload that is larger than MySQL text size (65,535 bytes, ~64kb). This patch (presumably) changes the column type to `mediumtext` with a size of `16,777,215 bytes, ~16MB).

I'm no rybyist so please double check the migration is all right.