Fix crash when a branch name starts with '/'

If a branch name starts with '/' it will be split into ['', ...] and
then mapped over with dot(), only dot() does not handle the empty
string. Teach dot() to handle the empty string.

This fixes the underlying problem in issue #91.

hg-fast-export.py

@@ -158,6 +158,7 @@ def sanitize_name(name,what="branch", mapping={}):
   # work to do manually, write a tool that does it for you.
 
   def dot(name):
+    if not name: return name
     if name[0] == '.': return '_'+name[1:]
     return name