pure parsers: properly detect corrupt index files

This new Python code should be equivalent in behavior to the if
statement at line 312 of parsers.c. Without this, the pure-python
parsers improperly ignore truncated revlogs as created in
test-verify.t.

mercurial/pure/parsers.py

@@ -56,6 +56,9 b' def parse_index2(data, inline):'
             n += 1
             off += s
 
+    if off != len(data):
+        raise ValueError('corrupt index file')
+
     if index:
         e = list(index[0])
         type = gettype(e[0])