convert: avoid svn log retrieval process cleanup

With huge history (like kdelibs), the process termination suddenly consumes a
lot of memory (from 700M to 1.3G+). Since the job is done, clean termination is
not required, just exit.

hgext/convert/subversion.py

@@ -95,6 +95,10 def get_log_child(fp, url, paths, start,
     else:
         pickle.dump(None, fp, protocol)
     fp.close()
+    # With large history, cleanup process goes crazy and suddenly
+    # consumes *huge* amount of memory. The output file being closed,
+    # there is no need for clean termination.
+    os._exit(0)
 
 def debugsvnlog(ui, **opts):
     """Fetch SVN log in a subprocess and channel them back to parent to