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>

git-rebase--interactive.sh

@@ -974,8 +974,9 @@ EOF
 
 		test -d "$REWRITTEN" || test -n "$NEVER_FF" || skip_unnecessary_picks
 
+		output git checkout $ONTO || die_abort "could not detach HEAD"
 		git update-ref ORIG_HEAD $HEAD
-		output git checkout $ONTO && do_rest
+		do_rest
 		;;
 	esac
 	shift

t/t3404-rebase-interactive.sh

@@ -146,6 +146,16 @@ test_expect_success 'abort' '
 	! test -d .git/rebase-merge
 '
 
+test_expect_success 'abort with error when new base cannot be checked out' '
+	git rm --cached file1 &&
+	git commit -m "remove file in base" &&
+	test_must_fail git rebase -i master > output 2>&1 &&
+	grep "Untracked working tree file .file1. would be overwritten" \
+		output &&
+	! test -d .git/rebase-merge &&
+	git reset --hard HEAD^
+'
+
 test_expect_success 'retain authorship' '
 	echo A > file7 &&
 	git add file7 &&