ui-log: Implement support for commit graphs

Teach CGit to print an ASCII art commit graph to the left of the commit
message, similar to 'git log --graph'. The graph adds extra lines (table
rows) to the log when needed to add/remove/shuffle edges in the graph.

When 'showmsg' is enabled, the graph is automatically padded to account
for the extra lines added by the commit message/notes.

This feature is controlled by a new config variable: "enable-commit-graph"
(disabled by default), and individual repos can control it by setting
"repo.enable-commit-graph".

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

cgit.css

@@ -153,6 +153,11 @@ table.list td {
 	padding: 0.1em 0.5em 0.1em 0.5em;
 }
 
+table.list td.commitgraph {
+	font-family: monospace;
+	white-space: pre;
+}
+
 table.list td.logsubject {
 	font-family: monospace;
 	font-weight: bold;
@@ -731,4 +736,4 @@ table.ssdiff td.space {
 
 table.ssdiff td.space div {
 	min-height: 3em;
-}
+}