Add .diff and .patch support to compare

[badhezi]

Implements proposal #34410

[wxiaoguang]

Some things in my mind:

1. It needs some tests to cover the behavior, otherwise this feature would get broken in the future.
2. It needs to handle edge cases like the.branch.name.contains.diff, the logic should be like this:
   • Parse {ref1}...{ref2}
   • If ref2 exists, then do not trim any suffix
   • If ref2 doesn't exist and it ends with .diff or .patch, then respond the diff/patch
3. Maybe it could add some dropdown menu items on the UI to make users could download the diff/patch by some clicks