support a revision-graph-specification language like that of git-rev-parse(1)

[Samuel Bronson]

I'd really like it if I could use a language something like the one used by git-rev-parse(1) <http://www.kernel.org/pub/software/scm/git/docs/git-rev-parse.html> with "bzr viz". The manpage has quite thorough documentation of the language, and is quite readable.

I'm especially interested in being able to specify *both* revspecs *and* branches, and in being able to exclude ancestors of a revision like with git's ^rev construct. The rev1..rev2 construct that desugars to rev1 ^rev2 would be nice as well.

I'm not too sure how what the best way for us to do allow mixed revids and branches, though, especially for those revspec forms that *refer* to a branch...