Auto merge of #11068 - arlosi:progress, r=epage

Change progress indicator for sparse registries

The progress indicator for sparse registries previously could go backwards as new dependencies are discovered, which confused users.

The new indicator looks like this:
```
    Updating crates.io index
       Fetch [====================>            ] 46 complete; 29 pending
```

The progress bar percentage is based the current depth in the dependency tree, with a hard coded limit at `10/11`. This provides natural feeling progress for many projects that I tested.

`complete` represents the number of index files downloaded, `pending` represents the number of index files that Cargo knows need to be downloaded but have not yet finished.

Fixes #10820