[MERGE] Add InterBranch.update_references()

[Jelmer Vernooij]

The attached patch moves the functionality of update_references() onto
InterBranch, allowing it to be overridden (as is necessary for bzr-svn
and bzr-git).

the per_interbranch tests are currently only run with a
non-reference-location branch format. Should I just make
GenericInterBranch._get_branch_formats_to_test() return two
BzrBranchFormat7 instances or rather add a separate explicit case in
bzrlib.tests.per_interbranch.__init__ ?

Cheers,

Jelmer
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