bookmarks: move parse() and current() into property definitions

bookmarks: refactor code responsible for updates of bookmarks introduce _bookmarksupdate for common parts from commitctx and addchangegroup

bookmarks: repo._bookmarkcurrent should be a propertycache

bookmarks: write() can simply access repo._bookmarks

bookmarks: turn repo._bookmarks into a propertycache

Merge with stable

hook: fix bug (reuse of variable) introduced in 872d49dd577a For binary installs, the 'name' argument would be reused as a local variable, destroying its original value. The patch fixes that, and also avoids copying sys.path when it's not necessary.