Drop --first-parent from rev count

There seems to be a problem with the policy of only traversing the first merge parent when determining the rev count portion of an incremental version number: it is not guaranteed that this count will be higher than the count of an ancestor commit. While the count is always higher “within a branch”, this does not necessarily hold when merging one branch into another: an old derivative branch may have a shorter line of development while pulling in a long list of changes from an upstream branch. In turn this means that incrementals:update sometimes selects a commit which looks newer than the current dependency version, and is an ancestor of the selected branch head, and yet which is not the maximal such commit topologically.