graph: wrap cascading commits after 4 columns

Currently the visual root commits in a graph cascade indefinitely until
a commit which is not a visual root or the last commit appears.
On filters like --author where one author might contribute mostly on
single patches this can become a visual issue.

Make the cascading wrap after 4 columns.

There are two possible cases of the wrap:

1. No ambiguity:

* A
  * B
    * C
      * D
* E
  * F

2. Ambiguous conflict:

If F happens to not be a visual root and E gets wrapped back to the
initial column then E and F would be vertically adjacent. The solution
is to forcefully indent E one level:

* A
  * B
    * C
      * D
  * E
* F
* F

The magic number 4 comes as the minimum number of columns to wrap where
the output shows clearly the commits are unrelated and doesn't cause too
much "pyramid" effects

Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

graph.c

diff --git a/graph.c b/graph.c
index 0870941..e3e2061 100644
--- a/graph.c
+++ b/graph.c
@@ -1042,6 +1042,23 @@ void graph_update(struct git_graph *graph, struct commit *commit)
 		 */
 		if (!graph->visual_root_depth && flags.is_next_visual_root)
 			graph->visual_root_cascade = 1;
+
+		/*
+		 * We wrap the cascading at a max of four columns at most, after
+		 * that we wrap it back to the initial column.
+		 *
+		 * This could cause ambiguity in case of the next commit not
+		 * being a visual root and be at the initial column after the
+		 * first wrap.
+		 *
+		 * In case of being a non-visual-root the next, stop the
+		 * cascading to get the commit indented.
+		 */
+		if (!flags.is_next_visual_root &&
+		    graph->visual_root_depth &&
+		    !(graph->visual_root_depth % 4))
+			graph->visual_root_cascade = 0;
+
 		graph->visual_root_depth++;
 	} else {
 		graph->visual_root_depth = 0;
@@ -1328,8 +1345,11 @@ static void graph_output_commit_line(struct git_graph *graph, struct graph_line
 				 * Each visual column is 2 characters wide.
 				 * Omit the indentation for the first visual
 				 * root in cascade mode.
+				 *
+				 * Have a max of 4 columns when cascading, after
+				 * that wrap it and repeat.
 				 */
-				int padding = (depth - graph->visual_root_cascade) * 2;
+				int padding = ((depth - graph->visual_root_cascade) % 4) * 2;
 				graph_line_addchars(line, ' ', padding);
 				graph->width += padding;
 			}

t/t4218-log-graph-indentation.sh

diff --git a/t/t4218-log-graph-indentation.sh b/t/t4218-log-graph-indentation.sh
index 60c7d84..d4c850c 100755
--- a/t/t4218-log-graph-indentation.sh
+++ b/t/t4218-log-graph-indentation.sh
@@ -511,4 +511,33 @@
 	EOF
 '
 
+# The cascading wraps after 4 columns and when wraping (column % 4 == 0) if the
+# next is a non visual-root, force indentation to avoid an ambiguous graph
+# (commit 59_A is forcefully indented)
+test_expect_success 'visual root cascading gets wrapped after 4 columns' '
+	create_orphan _58 && test_commit 58_A && test_commit 58_B &&
+	create_orphan _59 && test_commit 59_A &&
+	create_orphan _60 && test_commit 60_A &&
+	create_orphan _61 && test_commit 61_A &&
+	create_orphan _62 && test_commit 62_A &&
+	create_orphan _63 && test_commit 63_A &&
+	create_orphan _64 && test_commit 64_A &&
+	create_orphan _65 && test_commit 65_A &&
+	create_orphan _66 && test_commit 66_A &&
+	create_orphan _67 && test_commit 67_A &&
+	lib_test_check_graph _58 _59 _60 _61 _62 _63 _64 _65 _66 _67 <<-\EOF
+	* 67_A
+	  * 66_A
+	    * 65_A
+	      * 64_A
+	* 63_A
+	  * 62_A
+	    * 61_A
+	      * 60_A
+	  * 59_A
+	* 58_B
+	* 58_A
+	EOF
+'
+
 test_done