add Makefile fo graphics

grafiken/Makefile

 versionsgraph=\
-	ex1-graph.pdf
+	ex1-graph.pdf \
+	ex1-graph-precommit.pdf \
+	ex2-graph-precommit.pdf \
+	ex2-graph.pdf \
+	ex3-graph-premerge.pdf \
+	ex3-graph.pdf \
+	merge-fast-forward-premerge.pdf \
+	merge-fast-forward.pdf \
+	merge-three-way-premerge.pdf \
+	merge-three-way.pdf \
 
 all=\
 	$(versionsgraph)