Expose a BatchSize option...

Since 21f36fa47c79246afe9a150f3ca46b5afc0190fe, tfs changesets are fetched by batches of 100 changesets (instead of all at once) to better support big history when cloning (and use less memory).

This option let the user specify the size of the batch (because perhaps even 100 is too big).
Values accepted:
x < 0 => Only one batch with ALL the changesets
x == 0 => default value of 100
x > 0 => batches of x changesets
Let's not persist the value on fetch. Persisting it on clone is OK. I'd rather document that the config option is available, and allow temporary overrides via `--batch`.