update: wlock the repo for the whole 'hg update' command
The update command is touching the repository and should lock it for
the length of its operations. Equally importantly, it should lock the
repository when it is writing bookmarks. It wasn't doing so until now,
leaving doors open for all kinds of drunk beaver parties.
This results in some minor tests changes, and the fixing of a couple
of bugs from race conditions.
Code does not receive any changes beside extra indentation.