drawdag: update test repos by drawing the changelog DAG in ASCII Currently, we have "debugbuilddag" which is a powerful tool to build test cases but not intuitive. We may end up running "hg log" in the test to make the test more readable. This patch adds a "drawdag" extension with a "debugdrawdag" command for similar testing purpose. Unlike the cryptic "debugbuilddag" command, it reads an ASCII graph that is intuitive to human, so the test case can be more readable. Unlike "debugbuilddag", "drawdag" does not require an empty repo. So it can be used to add new changesets to an existing repo. Since the "drawdag" logic is not that trivial and only makes sense for testing purpose, the extension is added to the "tests" directory, to make the core logic clean. If we find it useful (for example, to demonstrate cases and help user understand some cases) and want to ship it by default in the future, we can move it to a ship-by-default "debugdrawdag" at that time.