feat: attach `git diff` in the PR description

[sasakisasaki]

Description

This PR fixes so the version update PR contains the git diff of the target repository. Currently the version update PR contains poor information in its description as this and this makes the review difficult.

Related links

Discussion for feature improvement

Tests performed

After the PR author finishes their tests, this will be updated.

I verified that a PR is created as this. The PR contains the git diff so I believe this is enough as tests.

Notes for reviewers

Currently, the version update PR does not contain git diff and the reviewer needs to go to the target repository by themselves. This PR reduces such the additional task.

Interface changes

No interface changes.

ROS Topic Changes

No ROS topic changes.

ROS Parameter Changes

No ROS parameter changes.

Effects on system behavior

The version update PR will be containing additional information (git diff).

Pre-review checklist for the PR author

The PR author must check the checkboxes below when creating the PR.

• [x] I've confirmed the contribution guidelines.
• [x] The PR follows the pull request guidelines.

In-review checklist for the PR reviewers

The PR reviewers must check the checkboxes below before approval.

• [ ] The PR follows the pull request guidelines.
• [ ] The PR has been properly tested.
• [ ] The PR has been reviewed by the code owners.

Post-review checklist for the PR author

The PR author must check the checkboxes below before merging.

• [ ] There are no open discussions or they are tracked via tickets.
• [ ] The PR is ready for merge.

After all checkboxes are checked, anyone who has write access can merge the PR.

[sasakisasaki]

Thank you for your proposals! Now let me do tests again with the fixed one.

[sasakisasaki]

@youtalk @xmfcx Now the version update PR is created as this. Hopefully this is the expected one :+1: .