convert/subversion: rehandle the no-tags case

Print meaningful error message if os.symlink fails

lock.py: cache hostname, but not pid, in case we fork

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.

Automated merge with http://hg.intevation.org/mercurial/crew

convert/subversion: Simplify callback.