Add a draft introduction to Git from graphs

images/git-graphs/repo.m4

@@ -0,0 +1,13 @@
+digraph repo {
+    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(`feature.dot')
+    include(`bugfix.dot')
+    m7 -> f1;
+    m9 -> b1;
+}