fileset: use UnknownIdentifier where appropriate

error: add a new UnknownIdentifier error type This will be used by the revset and fileset code to communicate more specific errors to higher levels so that we might be able to offer suggestions about what function might have been similar.

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.