Add tests for the graph
Improve error outputs
Add option to show a text graph
Add option to show a dot graph
Add option to show an xlib graph
Add usage to README.md
Add support for multiple upstream branches

Refactor of the graph
Branches are considered merged when they point to the same commmit as at
least one of their upstreams.
Reduce number of invocations to git checkout.
Generate README.md with m4