graph: add --graph-lane-limit option

Repositories that have many active branches at the same time produce
wide graphs. A lane consists of two columns, the edge and the space
padding, each branch takes a lane in the graph and there is no way
to limit how many can be shown.

Add '--graph-lane-limit=<n>' revision option that caps the number
of visible lanes to n. This option requires '--graph', without it
a limit to the graph has no meaning, in this case error out.

Zero and negative values are valid inputs but silently ignored
treating them as "no limit", the same as not using the option.
This follows what '--max-parents' does with negative values.

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

Author: pabloosabaterr

graph.c

@@ -317,6 +317,15 @@ struct git_graph {
 	struct strbuf prefix_buf;
 };
 
+static int graph_needs_truncation(struct git_graph *graph, int lane)
+{
+	int max = graph->revs->graph_max_lanes;
+	/*
+	 * Ignore values <= 0, meaning no limit.
+	 */
+	return max > 0 && lane >= max;
+}
+
 static const char *diff_output_prefix_callback(struct diff_options *opt, void *data)
 {
 	struct git_graph *graph = data;

revision.c

@@ -2605,6 +2605,8 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	} else if (!strcmp(arg, "--no-graph")) {
 		graph_clear(revs->graph);
 		revs->graph = NULL;
+	} else if (skip_prefix(arg, "--graph-lane-limit=", &optarg)) {
+		revs->graph_max_lanes = parse_count(optarg);
 	} else if (!strcmp(arg, "--encode-email-headers")) {
 		revs->encode_email_headers = 1;
 	} else if (!strcmp(arg, "--no-encode-email-headers")) {
@@ -3172,6 +3174,10 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 
 	if (revs->no_walk && revs->graph)
 		die(_("options '%s' and '%s' cannot be used together"), "--no-walk", "--graph");
+
+	if (revs->graph_max_lanes > 0 && !revs->graph)
+		die(_("option '%s' requires '%s'"), "--graph-lane-limit", "--graph");
+
 	if (!revs->reflog_info && revs->grep_filter.use_reflog_filter)
 		die(_("the option '%s' requires '%s'"), "--grep-reflog", "--walk-reflogs");
 

revision.h

@@ -304,6 +304,7 @@ struct rev_info {
 
 	/* Display history graph */
 	struct git_graph *graph;
+	int graph_max_lanes;
 
 	/* special limits */
 	int skip_count;