git-forensics fails with MissingObjectException when reference build contains removed commits

### Jenkins and plugins versions report

Environment

```text
Jenkins: 2.555.3
OS: Linux - 4.18.0-553.132.1.el8_10.x86_64
Java: 21.0.11 - Red Hat, Inc. (OpenJDK 64-Bit Server VM)
---
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apache-httpcomponents-client-4-api:4.5.14-269.vfa_2321039a_83
apache-httpcomponents-client-5-api:5.6.1-195.v65ffe15189a_d
asm-api:9.10.1-216.va_9256d3b_844b_
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favorite:2.263.v941d21defef7
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forensics-api:4.1891.v5e60f3377506
git:5.10.1
git-client:6.6.0
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ranorex-integration:1.0.2
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woodstox-core-api:7.1.1-1.v4d297985f397
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xray-connector:2.6.2

```

### What Operating System are you using (both controller, and any agents involved in the problem)?

Controller: Linux
Agent: Windows

### Reproduction steps

1. Configure a multibranch pipeline (GitLab MR setup) using Warnings NG + git-forensics:
```groovy
mineRepository(scm: scmId)

discoverGitReferenceBuild(
scm: scmId,
maxCommits: 1000,
skipUnknownCommits: true
)
2. Run builds normally to establish a baseline.
3. Perform a rebase or force-push that removes older commits from the repository history.
4. Trigger a new MR build.

### Expected Results

If a commit referenced by git-forensics no longer exists:

- the commit should be ignored
- delta computation should skip invalid commits or be disabled
- the analysis should continue without failing

### Actual Results

- Build fails during delta computation with:
```
org.eclipse.jgit.errors.MissingObjectException:
Missing unknown 123412351234
```
- The error occurs even though:
- the repository is valid (git fsck --full shows no issues)
- the commit cannot be fetched (git fetch origin fails)
- the workspace is freshly cloned

### Anything else?

- The missing commit is stored in the plugin’s internal state:
```
GitCommitsRecord (reference build #45):
latest: 123412351234
```
- This commit is no longer part of the Git repository (likely removed via rebase or force push).
- During delta computation (DeltaRepositoryCallback.calculateDelta()), the plugin attempts to resolve this commit via JGit without verifying its existence, leading to a crash.
- MR builds always fall back to the latest successful target branch build, even if that build references commits no longer present in the repository.
- skipUnknownCommits does not prevent this failure because it is not applied during delta calculation.

## Impact

- MR builds fail after rebases or force pushes
- Cleaning workspace and deleting builds does not resolve the issue
- Breaks normal Git workflows with rewritten history

## Related issues

This appears to be related to:
- https://github.com/jenkinsci/git-forensics-plugin/issues/1168

That issue describes incorrect behavior when a previously recorded commit no longer exists.
However, in this case, the same situation leads to a `MissingObjectException` during delta computation, causing build failure instead of degraded results.

### Are you interested in contributing a fix?

_No response_

Start at DeltaRepositoryCallback.calculateDelta() and inspect how the GitCommitsRecord for reference build #45 is resolved, using the reported MissingObjectException and the skipUnknownCommits behavior as context. Reproduce with a rebased or force-pushed repository, then verify that an unavailable commit is skipped or disables delta computation without failing the MR build.

Written by the indexing model from the issue text.