parsers: fix memleak of revlog cache entries on strip

Since 12a852c7c763, raw_length can be reduced on strip, but corresponding cache
entries still have refcount. They are not dereferenced by _index_clearcache(),
and never freed.

To reproduce the problem, run "hg pull" and "hg strip null" several times
in the same process.

mercurial/parsers.c

@@ -1234,8 +1234,14 b' static int index_slice_del(indexObject *'
 				self->ntrev = (int)start;
 		}
 		self->length = start + 1;
-		if (start < self->raw_length)
+		if (start < self->raw_length) {
+			if (self->cache) {
+				Py_ssize_t i;
+				for (i = start; i < self->raw_length; i++)
+					Py_CLEAR(self->cache[i]);
+			}
 			self->raw_length = start;
+		}
 		goto done;
 	}