CLOUD-869 Fallback to merge-base if commit hash is invalid

[ptankov]

CHANGE DESCRIPTION

Problem:
Sometimes git diff --name-only xxxxx HEAD can result in:

fatal: bad object xxxxx

Cause:
The commit is from a different branch or repository.
or
The commit history has been rewritten (e.g., through a rebase or force-push).

Solution:
Validate the commit hash before running git diff and fallback to merge-base if the commit is invalid.

CHECKLIST

Jira

• Is the Jira ticket created and referenced properly?
• Does the Jira ticket have the proper statuses for documentation (Needs Doc) and QA (Needs QA)?
• Does the Jira ticket link to the proper milestone (Fix Version field)?

Tests

• Is an E2E test/test case added for the new feature/change?
• Are unit tests added where appropriate?

Config/Logging/Testability

• Are all needed new/changed options added to default YAML files?
• Are all needed new/changed options added to the Helm Chart?
• Did we add proper logging messages for operator actions?
• Did we ensure compatibility with the previous version or cluster upgrade process?
• Does the change support oldest and newest supported PG version?
• Does the change support oldest and newest supported Kubernetes version?