Creates, updates and/or deletes multiple refs in a repository.

This mutation takes a list of RefUpdates and performs these updates
on the repository. All updates are performed atomically, meaning that
if one of them is rejected, no other ref will be modified.

RefUpdate.beforeOid specifies that the given reference needs to point
to the given value before performing any updates. A value of
0000000000000000000000000000000000000000 can be used to verify that
the references should not exist.

RefUpdate.afterOid specifies the value that the given reference
will point to after performing all updates. A value of
0000000000000000000000000000000000000000 can be used to delete a
reference.

If RefUpdate.force is set to true, a non-fast-forward updates
for the given reference will be allowed.