fix(api): handle fork-only commits in compare API (#37185)

Fix 500 error when comparing branches across fork repositories

## Problem

The compare API returns a 500 Internal Server Error when comparing
branches where the head commit exists only in the fork repository.

## Cause

The API was using the base repository's GitRepo and repository context
when converting commits. This fails when the commit does not exist in
the base repository, resulting in a "fatal: bad object" error.

## Solution

Use the head repository and HeadGitRepo when available to ensure commits
are resolved in the correct repository context.

## Result

* Fixes "fatal: bad object" error
* Enables proper comparison between base and fork repositories
* Prevents 500 Internal Server Error

Fixes #37168

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/api/v1/repo/compare.go

@@ -62,13 +62,20 @@ func CompareDiff(ctx *context.APIContext) {
 
 	apiCommits := make([]*api.Commit, 0, len(compareInfo.Commits))
 	userCache := make(map[string]*user_model.User)
+
 	for i := 0; i < len(compareInfo.Commits); i++ {
-		apiCommit, err := convert.ToCommit(ctx, ctx.Repo.Repository, ctx.Repo.GitRepo, compareInfo.Commits[i], userCache,
+		apiCommit, err := convert.ToCommit(
+			ctx,
+			compareInfo.HeadRepo,
+			compareInfo.HeadGitRepo,
+			compareInfo.Commits[i],
+			userCache,
 			convert.ToCommitOptions{
 				Stat:         true,
 				Verification: verification,
 				Files:        files,
-			})
+			},
+		)
 		if err != nil {
 			ctx.APIErrorInternal(err)
 			return