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
Please don't set the config option during fetch. If I do this:

    git config git-tfs.batch-size 500
    git tfs fetch --batch-size 100
    git tfs fetch

The second fetch should use a batch size of 500.