[confcom] Restore default behaviour of `--upload-fragment`

[DomAyre]

As of confcom 1.2.7, the default behaviour of --upload-fragment has changed in a breaking way.

Previously, it would attach the fragment to the first image specified in the input to the call, as of bca2ea1679e62d67a08edd9c4396d4d53b83c462, it now would push the fragment as a standalone fragment to a reference matching the feed (which would also break if the feed didn't happen to be a valid registry reference which is entirely possible).

We need to restore the default behaviour to avoid breaking users, while still keeping the ability to push standalone fragments.

[yonzhan]

Thank you for opening this issue, we will look into it.