org-clock: Fix number of time columns in clock table

* lisp/org-clock.el (org-clocktable-write-default): Limit number of
  time columns to the deepest headline level.

* testing/lisp/test-org-clock.el (test-org-clock/clocktable/maxlevel):
  Update tests.

lisp/org-clock.el

@@ -2474,8 +2474,20 @@ from the dynamic block definition."
 	 (level? (and (not compact?) (plist-get params :level)))
 	 (timestamp (plist-get params :timestamp))
 	 (properties (plist-get params :properties))
-	 (time-columns (if compact? 1
-			 (min maxlevel (or (plist-get params :tcolumns) 100))))
+	 (time-columns
+	  (if (or compact? (< maxlevel 2)) 1
+	    ;; Deepest headline level is a hard limit for the number
+	    ;; of time columns.
+	    (let ((levels
+		   (cl-mapcan
+		    (lambda (table)
+		      (pcase table
+			(`(,_ ,(and (pred wholenump) (pred (/= 0))) ,entries)
+			 (mapcar #'car entries))))
+		    tables)))
+	      (min maxlevel
+		   (or (plist-get params :tcolumns) 100)
+		   (if (null levels) 1 (apply #'max levels))))))
 	 (indent (or compact? (plist-get params :indent)))
 	 (formula (plist-get params :formula))
 	 (case-fold-search t)

testing/lisp/test-org-clock.el

@@ -370,12 +370,12 @@ CLOCK: [2012-03-29 Thu 16:40]--[2014-03-04 Thu 00:41] => 16905:01
 (ert-deftest test-org-clock/clocktable/maxlevel ()
   "Test \":maxlevel\" parameter in Clock table."
   (should
-   (equal "| Headline     | Time   |      |   |
-|--------------+--------+------+---|
-| *Total time* | *6:00* |      |   |
-|--------------+--------+------+---|
-| Foo          | 6:00   |      |   |
-| \\_  Bar      |        | 2:00 |   |
+   (equal "| Headline     | Time   |      |
+|--------------+--------+------|
+| *Total time* | *6:00* |      |
+|--------------+--------+------|
+| Foo          | 6:00   |      |
+| \\_  Bar      |        | 2:00 |
 "
           (org-test-with-temp-text
               "