graph: add documentation and tests about --graph-lane-limit

Document --graph-lane-limit option in rev-list-options.adoc with
--graph option.

Add two tests in t4215 reusing existing graphs. The first test
limits to two lanes on a single tip and the second one limits to
three lanes on multiple tips. Both tests check that everything
is truncated correctly.

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

Author: pabloosabaterr

Documentation/rev-list-options.adoc

@@ -1259,6 +1259,11 @@ This implies the `--topo-order` option by default, but the
 	in between them in that case. If _<barrier>_ is specified, it
 	is the string that will be shown instead of the default one.
 
+`--graph-lane-limit=<n>`::
+	When `--graph` is used, limit the number of graph lanes to be shown.
+	Lanes over the limit are replaced with a truncation mark '.'. By default
+	there is no limit.
+
 ifdef::git-rev-list[]
 `--count`::
 	Print a number stating how many commits would have been

t/t4215-log-skewed-merges.sh

@@ -370,4 +370,57 @@ test_expect_success 'log --graph with multiple tips' '
 	EOF
 '
 
+test_expect_success 'log --graph --graph-lane-limit=2 limited to two lanes' '
+	check_graph --graph-lane-limit=2 M_7 <<-\EOF
+	*-.   7_M4
+	|\ \
+	| | * 7_G
+	| | * 7_F
+	| * . 7_E
+	| * . 7_D
+	* | . 7_C
+	| |/
+	|/|
+	* | 7_B
+	|/
+	* 7_A
+	EOF
+'
+
+test_expect_success 'log --graph --graph-lane-limit=3 limited to three lanes' '
+	check_graph --graph-lane-limit=3 M_1 M_3 M_5 M_7 <<-\EOF
+	*   7_M1
+	|\
+	| | *   7_M2
+	| | |\
+	| | | * 7_H
+	| | | . 7_M3
+	| | | . 7_J
+	| | | . 7_I
+	| | | . 7_M4
+	| |_|_.
+	|/| | .
+	| | |_.
+	| |/| .
+	| | | .
+	| | |/.
+	| | * . 7_G
+	| | | .
+	| | |/.
+	| | * . 7_F
+	| * | . 7_E
+	| | |/.
+	| |/| .
+	| * | . 7_D
+	| | |/
+	| |/|
+	* | | 7_C
+	| |/
+	|/|
+	* | 7_B
+	|/
+	* 7_A
+	EOF
+'
+
 test_done