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.
strip: hold wlock for entire duration
Previously, we'd acquire and release the wlock several times. This meant that
other hg processes could come in and change state. Instead of that, retain the
wlock for the entire duration of the strip.