Add a draft introduction to Git from graphs

images/git-graphs/repo_labels_alice.m4

@@ -0,0 +1,37 @@
+digraph repo_labels {
+    rankdir = "LR";
+    bgcolor = "transparent";
+    node[width=0.15, height=0.15, shape=point, color=white, fontcolor=white];
+    edge[weight=2, arrowhead=normal, arrowsize=0.6, color=white];
+    define(`digraph', `subgraph')
+    include(`master.dot')
+    include(`merged.dot')
+    include(`bugfix.dot')
+    m9 -> b1;
+    include(`feature.dot')
+    m7 -> f1;
+    f3 -> f4;
+
+    node[color=green];
+    edge[color=green];
+    node[group=feature];
+    bobf1 -> bobf2 -> bobf3 -> bobf4;
+    m7 -> bobf1;
+
+    node[shape=box, color=white, fontname="monospace"];
+    edge[color=white];
+    subgraph labels {
+	rank = "max";
+	//HEAD [style=dashed];
+	master;
+	"bob/feature";
+	feature;
+	bugfix;
+    }
+    edge[arrowhead=none, style=dashed];
+    //m10 -> HEAD;
+    m10 -> master;
+    bobf4 -> "bob/feature";
+    f4 -> feature;
+    b2 -> bugfix;
+}