Refactor getpatch/getdiff functions and remove unnecessary fallback (#32817)

Extract from #32786 

`git diff a..b` is equal to `git diff a b` which is different from `git
diff a...b`. For pull request, we should always 

---------

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

modules/git/repo_compare_test.go

@@ -28,7 +28,7 @@ func TestGetFormatPatch(t *testing.T) {
 	defer repo.Close()
 
 	rd := &bytes.Buffer{}
-	err = repo.GetPatch("8d92fc95^", "8d92fc95", rd)
+	err = repo.GetPatch("8d92fc95^...8d92fc95", rd)
 	if err != nil {
 		assert.NoError(t, err)
 		return