[RFC] Incremental svn_delta_path_driver

Does the attached patch look reasonable? It provides an incremental version of the existing svn_delta_path_driver2() function, in which the caller can provide one path at a time instead of all at once at the start.

We have a few (5 ~ 10) uses of svn_delta_path_driver2() in our codebase and I think a few more places already where an incremental path driver could be used if it existed, in addition to me now wanting to use it in "unshelve" as I mentioned the other day. I will try doing that, and, if/when working usefully, will commit it, along with any changes suggested here.

It already works as a factored-out body of svn_delta_path_driver2(): tests pass with this patch applied.

In svn_delta_path_driver2() there is some special handling of the root path, which we probably want to include in the incremental driver too; I just haven't got around to studying and deciding on that part yet.

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- Julian