"backend" commit slows create operations

[mih]

Dataset create is much slower than what one assume it to be given the plain Git analogs that come to mind. One reason that the any standard dataset creation involves at least two commits. The first one is happening in the AnnexRepo constructor to set a default backend.

Alternatively, we could "manually" set the backend via AnnexRepo.set_default_backend(commit=False)
after a repo has been created, and handle this changeset in the final commit.

Ping #2908

[mih]

Turns on the impact is only ~10ms. But the cosmetics are better with a single commit that comprises the entire initial setup.

[mih]

With #3383 merged, this is now fixed in master.