The different branching approaches makes migrating history difficult. Microsoft's recommendation is to migrate just the tip, but will allow you to migrate up to 180 days of history for the trunk (no branches).

If you need branch history, or more than 180 days worth of history, the only option is third-party tooling (git-tfs). It seems to work good enough for development purposes (i.e. git blame)... but I'm not sure if it's good enough if we need the history for legal purposes.