rhg: fallback when encountering ellipsis revisions

Ellipsis revisions are still experimental and buggy in non-trivial
histories. We currently have no plans to improve this situation nor
to add support for ellipsis revisions in `rhg`.

Falling back should be done carefully (since we may have already done
some work that is visible to the user), but in this case it's highly
unlikely that we're doing anything useful with a repo with ellipsis
revisions, so this should be strictly better, also since the
error message is more explicit.

rust/hg-core/src/revlog/revlog.rs

@@ -447,6 +447,11 b" impl<'a> RevlogEntry<'a> {"
         ) {
             Ok(data)
         } else {
+            if (self.flags & REVISION_FLAG_ELLIPSIS) != 0 {
+                return Err(HgError::unsupported(
+                    "ellipsis revisions are not supported by rhg",
+                ));
+            }
             Err(corrupted(format!(
                 "hash check failed for revision {}",
                 self.rev