montone: fix addition to list by using .append() instead of '+'

Differential Revision: https://phab.mercurial-scm.org/D5739

hgext/convert/monotone.py

@@ -102,7 +102,7 b' class monotone_source(common.converter_s'
 
         command.append('l')
         for arg in args:
-            command += "%d:%s" % (len(arg), arg)
+            command.append("%d:%s" % (len(arg), arg))
         command.append('e')
         command = ''.join(command)