Batch reference updates together for storage

clone, fetch and push can all update multiple references in a single
command invocation. Rather than performing sequential iteration
of each reference change inside of the application code, push this
down into the reference database where the implementation can take
advantage of the batch size and optimize itself.

For the local filesystem implementation the obvious optimization
is to write a packed-refs file when the repository is completely
empty. The initial clone, fetch or push into the destination may
have hundreds of new references. Writing all of these as loose
files is not efficient. This optimization is not implemented in
this commit and is left as an exercise for the reader to supply
in a future commit to JGit.

To make the API changes simple, define the BatchRefUpdate type and
implementation using the existing sequential behavior.

Change-Id: I8e1674f091e05e24e3ff56ccbc687a6d18a6a61e

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