Bug: ci: use git diff to add `M-database-changes` label for pr that contains migrations and schema changes



Use git diff to add `M-database-changes` label for pr that contains migrations and schema changes. This helps to identify the pr that has db migrations.
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [x] CI/CD

## Description
<!-- Describe your changes in detail -->
This ps uses git diff to add `M-database-changes` label for pr that contains migrations and schema changes.

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
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Why is this change required? What problem does it solve?
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## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
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We can see that `M-database-changes` was added for on of the below commits which contained migration changes.
<img width="1050" alt="image" src="https://github.com/juspay/hyperswitch/assets/83439957/959f47f3-622b-40ef-a28d-2ba76dd51d9f">


## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
