Audit the APR pool usage of the recursive function get_mergeinfo_for_path() (issue #3025).
* subversion/libsvn_fs_util/mergeinfo-sqlite-index.c Include apr.h for (at least) apr_ssize_t. (get_mergeinfo_for_path): Add documentation indicating that a sub-pool should be used with this function. (index_path_mergeinfo, get_mergeinfo): Introduce sub-pool for use with the call to get_mergeinfo_for_path(), dup'ing the returned mergeinfo hash into POOL. (mergeinfo_hash_dup): Add new function that returns a deep copy of a hash of paths -> mergeinfo hashes.