Try ditaa. It can transform any ASCII diagram into an image. Although is was not designed with Git branches in mind, I was impressed by the results.
Source (txt file):
+--------+
| hotfix |
+---+----+
|
--*<---*<---*
^
|
\--*<---*
|
+---+----+
| master |
+--------+
Command:
java -jar ditaa0_9.jar ascii-graph.txt
Result:
It also supports background colors, dashed lines, different shapes and more. See the examples.