Bug: oras.Copy() may skip tagging if manifest blob already exists in the destination

[qweeah]

Reproduce Step

oras CLI version: 0.13.0

1. Package an OCI artifact and push
   echo "test" > hi.txt && oras push $REG/$REPO hi.txt
2. Repush the artifact
   oras push $REG/$REPO:tag hi.txt
3. Push will be successful on client side but tag won't be created in the registry.

Root Cause

If a manifest blob is found existed (https://github.com/oras-project/oras-go/blob/main/copy.go#L154-L163), the copy process will be skipped as well as the retagging process.

Originally posted by @qweeah in oras-project/oras#447 (comment)

[Wwwsylvia]

I will look into this next week.

[m5i-work]

Please load balance to me.

[Wwwsylvia]

Thanks @m5i-work. Assigned.

[Wwwsylvia]

Hi @jpower432, FYI, this bug has been fixed in the latest main branch.