- use "branch 'foo'" to distinguish from "branch merge".
- commit messags can be empty (to abort commits)
- Added value for editor message: Tell about HG: lines like CVS does.
The main problem was that dirstate.getcwd() returned just "",
which was interpreted as "we're at the repo root". It now returns
an absolute path.
The util.pathto function was also changed to deal with the "cwd is
an absolute path" case.
