ui-log: Colorize commit graph

Use the existing coloring logic in Git's graph code to color the lines
between commits in the commit graph.

Whereas Git normally uses ANSI color escapes to produce colors, we here
use graph_set_column_colors() to replace those with HTML color escapes,
that embed the graph lines in <span> tags that apply the desired color
using CSS.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.css

@@ -158,6 +158,30 @@ table.list td.commitgraph {
 	white-space: pre;
 }
 
+table.list td.commitgraph .column1 {
+	color: #a00;
+}
+
+table.list td.commitgraph .column2 {
+	color: #0a0;
+}
+
+table.list td.commitgraph .column3 {
+	color: #aa0;
+}
+
+table.list td.commitgraph .column4 {
+	color: #00a;
+}
+
+table.list td.commitgraph .column5 {
+	color: #a0a;
+}
+
+table.list td.commitgraph .column6 {
+	color: #0aa;
+}
+
 table.list td.logsubject {
 	font-family: monospace;
 	font-weight: bold;

ui-log.c

@@ -13,6 +13,21 @@
 
 int files, add_lines, rem_lines;
 
+/*
+ * The list of available column colors in the commit graph.
+ */
+static const char *column_colors_html[] = {
+	"<span class='column1'>",
+	"<span class='column2'>",
+	"<span class='column3'>",
+	"<span class='column4'>",
+	"<span class='column5'>",
+	"<span class='column6'>",
+	"</span>",
+};
+
+#define COLUMN_COLORS_HTML_MAX (ARRAY_SIZE(column_colors_html) - 1)
+
 void count_lines(char *line, int size)
 {
 	if (size <= 0)
@@ -273,7 +288,11 @@ void cgit_print_log(const char *tip, int ofs, int cnt, char *grep, char *pattern
 	}
 	if (ctx.repo->enable_commit_graph) {
 		static const char *graph_arg = "--graph";
+		static const char *color_arg = "--color";
 		vector_push(&vec, &graph_arg, 0);
+		vector_push(&vec, &color_arg, 0);
+		graph_set_column_colors(column_colors_html,
+					COLUMN_COLORS_HTML_MAX);
 	}
 
 	if (path) {