tag: move the prohibition of tagging the `null` rev up to the `wdir()` check It makes sense to do these together, and avoid another revision lookup.

branching: merge with default the first part of the fix is there

dirstate: handle missing backup file on restoration This is the stable counter part to e358f6e0e50e. Since 6.4 will stop writing undo.dirstate in some case (actually… at all), a transaction created with 6.4 and recover/rolledback with 6.3 need to work to a certain degreee. This changeset add the necessary bits so that we don't get a traceback from 6..3 in this cases.