feat: add raw diff/patch endpoint for repository comparisons (#37632)

## Summary

Adds `GET
/repos/{owner}/{repo}/compare/{basehead}.{diffType:diff|patch}`,
mirroring the existing `/git/commits/{sha}.{diffType}` endpoint but for
comparisons between two arbitrary refs.

The new endpoint streams a raw unified diff or `git format-patch` output
between any two refs:

GET /repos/{owner}/{repo}/compare/main...feature.diff
GET /repos/{owner}/{repo}/compare/v1.0..v1.1.patch
GET /repos/{owner}/{repo}/compare/abc1234...def5678.diff

Resolves #5561, #13416 and #17165.

AI was used while creating this PR. Automated tests were added as per
the contribution policy.

---------

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

routers/web/repo/pull.go

@@ -1310,7 +1310,7 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 	form := web.GetForm(ctx).(*forms.CreateIssueForm)
 	repo := ctx.Repo.Repository
 	comparePageInfo := newComparePageInfo()
-	err := comparePageInfo.parseCompareInfo(ctx)
+	err := comparePageInfo.parseCompareInfo(ctx, ctx.PathParam("*"))
 	if errors.Is(err, util.ErrNotExist) {
 		ctx.JSONErrorNotFound()
 		return