commands.bookmark: use unsetcurrent instead of setcurrent with None
There are currently two different ways we can have no active bookmark:
.hg/bookmarks.current being missing and it being an empty file. This patch and
upcoming ones make an empty file the only way to represent no active bookmarks.
This is the right choice because it matches the state that a new repository
without bookmarks will be in.
workingfilectx.date = (1000, 0)
ASCII : Gr?ezi!
Latin-1 : Grüezi!
UTF-8 : Grüezi!