When comparing with an non-exist repository, return 404 but 500 (#27437) (#27442)

Backport #27437 by @lunny

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/repo/compare.go

@@ -252,7 +252,6 @@ func ParseCompareInfo(ctx *context.Context) *CompareInfo {
 		isSameRepo = true
 		ci.HeadUser = ctx.Repo.Owner
 		ci.HeadBranch = headInfos[0]
-
 	} else if len(headInfos) == 2 {
 		headInfosSplit := strings.Split(headInfos[0], "/")
 		if len(headInfosSplit) == 1 {
@@ -407,6 +406,9 @@ func ParseCompareInfo(ctx *context.Context) *CompareInfo {
 			return nil
 		}
 		defer ci.HeadGitRepo.Close()
+	} else {
+		ctx.NotFound("ParseCompareInfo", nil)
+		return nil
 	}
 
 	ctx.Data["HeadRepo"] = ci.HeadRepo