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.
