Use batch database operations instead of one by one to optimze api pulls (#32680)

Resolve #31492

The response time for the Pull Requests API has improved significantly,
dropping from over `2000ms` to about `350ms` on my local machine. It's
about `6` times faster.

A key area for further optimization lies in batch-fetching data for
`apiPullRequest.ChangedFiles, apiPullRequest.Additions, and
apiPullRequest.Deletions`.

Tests `TestAPIViewPulls` does exist and new tests added.

- This PR also fixes some bugs in `GetDiff` functions.
- This PR also fixes data inconsistent in test data. For a pull request,
the head branch's reference should be equal to the reference in
`pull/xxx/head`.

routers/api/v1/repo/pull.go

@@ -139,43 +139,12 @@ func ListPullRequests(ctx *context.APIContext) {
 		return
 	}
 
-	apiPrs := make([]*api.PullRequest, len(prs))
-	// NOTE: load repository first, so that issue.Repo will be filled with pr.BaseRepo
-	if err := prs.LoadRepositories(ctx); err != nil {
-		ctx.Error(http.StatusInternalServerError, "LoadRepositories", err)
-		return
-	}
-	issueList, err := prs.LoadIssues(ctx)
+	apiPrs, err := convert.ToAPIPullRequests(ctx, ctx.Repo.Repository, prs, ctx.Doer)
 	if err != nil {
-		ctx.Error(http.StatusInternalServerError, "LoadIssues", err)
+		ctx.Error(http.StatusInternalServerError, "ToAPIPullRequests", err)
 		return
 	}
 
-	if err := issueList.LoadLabels(ctx); err != nil {
-		ctx.Error(http.StatusInternalServerError, "LoadLabels", err)
-		return
-	}
-	if err := issueList.LoadPosters(ctx); err != nil {
-		ctx.Error(http.StatusInternalServerError, "LoadPoster", err)
-		return
-	}
-	if err := issueList.LoadAttachments(ctx); err != nil {
-		ctx.Error(http.StatusInternalServerError, "LoadAttachments", err)
-		return
-	}
-	if err := issueList.LoadMilestones(ctx); err != nil {
-		ctx.Error(http.StatusInternalServerError, "LoadMilestones", err)
-		return
-	}
-	if err := issueList.LoadAssignees(ctx); err != nil {
-		ctx.Error(http.StatusInternalServerError, "LoadAssignees", err)
-		return
-	}
-
-	for i := range prs {
-		apiPrs[i] = convert.ToAPIPullRequest(ctx, prs[i], ctx.Doer)
-	}
-
 	ctx.SetLinkHeader(int(maxResults), listOptions.PageSize)
 	ctx.SetTotalCountHeader(maxResults)
 	ctx.JSON(http.StatusOK, &apiPrs)