commands.bookmark: use unsetcurrent instead of setcurrent with None

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.

--- a/mercurial/commands.py	Fri Nov 22 17:26:58 2013 -0600
+++ b/mercurial/commands.py	Fri Nov 15 18:06:01 2013 -0800
@@ -865,7 +865,7 @@
             if mark not in marks:
                 raise util.Abort(_("bookmark '%s' does not exist") % mark)
             if mark == repo._bookmarkcurrent:
-                bookmarks.setcurrent(repo, None)
+                bookmarks.unsetcurrent(repo)
             del marks[mark]
         marks.write()
 
@@ -891,7 +891,7 @@
             if newact is None:
                 newact = mark
             if inactive and mark == repo._bookmarkcurrent:
-                bookmarks.setcurrent(repo, None)
+                bookmarks.unsetcurrent(repo)
                 return
             tgt = cur
             if rev:
@@ -901,7 +901,7 @@
         if not inactive and cur == marks[newact] and not rev:
             bookmarks.setcurrent(repo, newact)
         elif cur != tgt and newact == repo._bookmarkcurrent:
-            bookmarks.setcurrent(repo, None)
+            bookmarks.unsetcurrent(repo)
         marks.write()
 
     # Same message whether trying to deactivate the current bookmark (-i
@@ -913,7 +913,7 @@
         if not repo._bookmarkcurrent:
             ui.status(_("no active bookmark\n"))
         else:
-            bookmarks.setcurrent(repo, None)
+            bookmarks.unsetcurrent(repo)
 
     else: # show bookmarks
         for bmark, n in sorted(marks.iteritems()):