shelve: use rebase instead of merge (issue4068)

Previously, shelve used merge to unshelve things. This meant that if you shelved
changes on one branch, then unshelved on another, all the changes from the first
branch would be present in the second branch, and not just the shelved changes.

The fix is to use rebase to pick the shelve commit off the original branch and
place it on top of the new branch. This means only the shelved changes are
brought across.

This has the side effect of fixing several other issues in shelve:

- you can now unshelve into a file that already has pending changes
- unshelve a mv/cp now has the correct dirstate value (A instead of M)
- you can now unshelve to an ancestor of the shelve
- unshelve now no longer deletes untracked .orig files

Updates tests and adds a new one to cover the issue. The test changes fall into
a few categories:

- I removed some excess output
- The --continue/--abort state is a little different, so the parents and
  dirstate needed updating
- Removed some untracked files at certain points that cluttered the output

--HG--
branch : stable