log: display closing-branch nodes as "_" (BC)
In plain `hg log` there is no indication that a commit closes a
branch. You can use hg log --debug, but this is too verbose. A simple
idea copied from thg and other graphical viewers is to display the
node for a closing-branch commit as a horizontal line.
I think this technically is a BC if we consider the graphlog to be
part of the stdout API, but I really can't imagine who the hell is
parsing the graphlog to determine information about commits.