fuzz: exercise more of the revlog API

I noticed in the coverage report that we didn't have much coverage in
revlog.py. Let's try and get some of the more interesting bits tested
by the fuzzer. I ran this locally for a few minutes to verify that I
appear to be calling the various functions in reasonable ways.

Differential Revision: https://phab.mercurial-scm.org/D5641

contrib/fuzz/revlog.cc

@@ -19,6 +19,11 b' from parsers import parse_index2'
 for inline in (True, False):
     try:
         index, cache = parse_index2(data, inline)
+        index.slicechunktodensity(list(range(len(index))), 0.5, 262144)
+        for rev in range(len(index)):
+            node = index[rev][7]
+            partial = index.shortest(node)
+            index.partialmatch(node[:partial])
     except Exception as e:
         pass
         # uncomment this print if you're editing this Python code