bookmarks: update and updatecurrentbookmark return status
This makes bookmarks.update() and bookmarks.updatecurrentbookmark() return
True or False to indicate whether the bookmark was updated or not. This allows
callers to e.g. abort if the update failed.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24