rebase -i: Abort cleanly if new base cannot be checked out

rebase -i: Abort cleanly if new base cannot be checked out

Untracked content in the working tree may prevent rebase -i from checking out
the new base onto which it wants to replay commits, if the new base commit
includes files at those (now untracked) paths. Currently, rebase -i dies
uncleanly in this situation, updating ORIG_HEAD and leaving a useless
.git/rebase-merge directory, with which the user can do nothing useful except
rebase --abort. Make rebase -i abort the procedure itself instead, as
non-interactive rebase already does, and add a test for this behavior.

Signed-off-by: Ian Ward Comfort <icomfort@stanford.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>