MergeImageInfo command does not have any useful output

The output of a successful run of the `MergeImageInfo` command looks like this:

```
READING MANIFEST
----------------
```

(from [this build](https://dev.azure.com/dnceng/internal/_build/results?buildId=2702498&view=logs&j=739bb7ec-7296-5b40-7dff-ee3297bfdaa4&t=e9450724-223d-5c25-ea90-d48101df80fb))

Surely there should be _some_ additional diagnostic information we can log so that it's obvious the command is doing something. Some examples:

- List which imageInfo files were found in the input folder
- List which contents were determined to be out of date content and thus were removed
- Some output for each object that gets merged
- Writing the output to the disk