merge default into stable for 3.7 code freeze

shelve: permit shelves to contain unknown files If an emergency comes in while you're in the middle of an experimental change, it can be useful to shelve not just files hg already tracks but also your unknown files while you handle the emergency. This is especially true if you have hooks intended to prevent you from forgetting to add new code before you push. Teach "hg shelve" to optionally shelve unknown files, not just tracked files. This is functionally similar to --addremove, but with two differences: 1) Deleted files are not removed. 2) Files added during shelve creation are tracked in extra so that they can be forgotten by "hg unshelve". When unshelving, we take care to only forget files if they've been created during the unshelve operation; if you add a file that's being tracked in a shelve as an unknown file, it should not become unknown again when the shelve is unshelved.