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 for some big repositories).

Values accepted:
x < 0 => Only one batch with ALL the changesets
x == 0 => default value of 100
x > 0 => batches of x changesets