Allow utf8 in mappings

We were previously processing entries in mapping files (when
`--mappings-are-raw` is not given) with
`.decode('unicode_escape').encode('utf8')` to replace backslash escape
sequences in bytestrings with the utf-8 encoded characters they
represent. However, it turns out that `.decode
('unicode_escape')` assumes latin-1 encoding if it encounters non-ascii
bytes: https://bugs.python.org/issue21331. So this gave incorrect
results if non-ascii utf8 data was present in the mapping.

To fix this, we now add an extra layer of `.decode('utf8').encode
('unicode-escape')` in order to convert any non-ascii characters into
their backslash escape sequences. Then the subsequent
`.decode('unicode_escape')` only encounters ascii characters and gives
correct results.

hg-fast-export.py

@@ -426,12 +426,20 @@ def load_mapping(name, filename, mapping_is_raw):
       return None
     return (m.group(1).strip(), m.group(2).strip())
 
+  def process_unicode_escape_sequences(s):
+    # Replace unicode escape sequences in the otherwise UTF8-encoded bytestring s with
+    # the UTF8-encoded characters they represent. We need to do an additional
+    # .decode('utf8').encode('unicode-escape') to convert any non-ascii characters into
+    # their escape sequences so that the subsequent .decode('unicode-escape') succeeds:
+    return s.decode('utf8').encode('unicode-escape').decode('unicode-escape').encode('utf8')
+
   def parse_quoted_line(line):
     m=quoted_regexp.match(line)
     if m==None:
-      return None
-    return (m.group(1).decode('unicode_escape').encode('utf8'),
-            m.group(5).decode('unicode_escape').encode('utf8'))
+      return 
+    
+    return (process_unicode_escape_sequences(m.group(1)),
+            process_unicode_escape_sequences(m.group(5)))
 
   cache={}
   if not os.path.exists(filename):