canonic(): don't use abspath() for filenames looking like <...>; this

fixes the problem reported in SF bug #477023 (Jonathan Mark): "pdb:
unexpected path confuses Emacs".

Lib/bdb.py

@@ -23,6 +23,8 @@ def __init__(self):
         self.fncache = {}
 
     def canonic(self, filename):
+        if filename == "<" + filename[1:-1] + ">":
+            return filename
         canonic = self.fncache.get(filename)
         if not canonic:
             canonic = os.path.abspath(filename)