Improving performance enhancement specifically during diff stage

[rohitagg2020]

Improving performance enhancement specifically during diff stage.

By go profiling, we have noticed that there are too many calls to deepCopy and AsYAMLBytes

Improvements done:

1. While rebas'ing a resource, we wont do Deepcopy until the resource being rebased matches with the resource for which rebase resource is defined.
2. default.go which was already in yaml was unnecessary being converted to struct and then back to yaml. Removed this unnecessary conversion.
3. Reduced calls to AsYAMLBytes.

Only one breaking scenario w.r.t. changes proposed:

1. kapp deploy (config with deployment) with v0.15.0 or earlier. Most of the times, revision is not set while storing md5. But sometimes it can. Lets move forward with scenario where it can.
2. There has been no changes.
3. kapp deploy (config with deployment) with these changes. In this case, the user will see the diff’s.

The chances of this scenario are extremely low…

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

[kumaritanushree]

LGTM