This fixes two minor problems with the prompts:
1. When pressing shift-A in the local commits view, it would first prompt
whether to stage all files, and then it would prompt whether to amend the
commit at all. This doesn't make sense, it needs to be the other way round.
2. When pressing shift-A on the head commit in an interactive rebase, we would
ask whether they want to amend the last commit, like when pressing shift-A in
the files view. While this is technically correct, the fact that we're
amending the head commit in this case is just an implementation detail, and
from the user's point of view it's better to use the same prompt as we do for
any other commit.
To fix these, we remove the confirmation panel from AmendHelper.AmendHead() and
instead add it at the two call sites, so that we have more control over this.
