Feat: add commit amend & Force push

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What does this MR do and why?

1. Implement getCommit (call get single commit from commitRESTAPI)
2. implement getCommitDiff (call get commit diff from commitRESTAPI)
3. use both methods to implement amend logic
4. Implemented amend logic
   1. Merge commit diff with current changes
   2. current changes will override commit diff if there is a conflict

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related

#482 (closed)

Screenshots or screen recordings

Test-amend-3

How to set up and validate locally

Numbered steps are found in https://gitlab.com/gitlab-org/gitlab-web-ide/-/blob/main/docs/contributing/development-environment-setup.md