API to get revision size on disk

I am working on an API to report the size on disk of a given revision, in FSFS. A client wants this for (human) informational purposes -- so users or administrators can understand how much space their commits are using.

Before FSFS f7, this information was easy to obtain from the on-disk size of a revision file, or for a packed shard by taking (offset[REV] - offset[REV-1]) from the manifest file. Since f7 it's harder, when revisions are packed.

This seems a reasonable feature to me.

The attached patch starts to implement it, providing an API in this form which is like a cut-down version of svn_fs_fs__get_stats():

/* Set *REV_SIZE to the total size of objects belonging to revision REVISION
 * in FS. */
svn_error_t *
svn_fs_fs__get_rev_size(apr_off_t *rev_size,
                        svn_fs_t *fs,
                        svn_revnum_t revision,
                        apr_pool_t *scratch_pool);

and a CLI in this form:

    $ svnfsfs rev-size /path/to/repo -r1
    1238 bytes in revision 1

The definition of "size" I have chosen excludes the index entries, because determining the on-disk size of them is tricky (they are stored in a variable-length encoding) and their size is not particularly important for the purposes.

TODO before commit:
  * support FSFS pre-f7 and physical addressing mode
  * simplify implementation by using the existing svn_fs_fs__dump_index() API with a simple callback

Any comments?

- Julian