give correct error msg

Author: HesterG

routers/web/repo/compare.go

@@ -732,14 +732,14 @@ func CompareDiff(ctx *context.Context) {
 
 	headBranches, _, err := ci.HeadGitRepo.GetBranchNames(0, 0)
 	if err != nil {
-		ctx.ServerError("GetTagNamesByRepoID", err)
+		ctx.ServerError("GetBranches", err)
 		return
 	}
 	ctx.Data["HeadBranches"] = headBranches
 
 	headTags, err := repo_model.GetTagNamesByRepoID(ctx, ci.HeadRepo.ID)
 	if err != nil {
-		ctx.ServerError("GetTags", err)
+		ctx.ServerError("GetTagNamesByRepoID", err)
 		return
 	}
 	ctx.Data["HeadTags"] = headTags