convert/subversion: work around memory leak in svn's python bindings
The svn.ra.get_log wrapper attaches the hash of changed paths for every
log entry to a global memory pool, so memory consumption increases
rapidly, with no way to free it.
Our workaround is to call this function in a child process, and feed
its results back over a pipe. The memory consumption of the child still
grows huge (hundreds of megabytes), but at least it goes away once the
reading-the-log phase is done.