rebase -i: do not fail when there is no commit to cherry-pick

In case there is no commit to apply (for example because you rebase to
upstream and all your local patches have been applied there), do not
fail.  The non-interactive rebase already behaves that way.

Do this by introducing a new command, "noop", which is substituted for
an empty commit list, so that deleting the commit list can still abort
as before.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-rebase--interactive.sh

@@ -277,7 +277,7 @@ do_next () {
 		"$DOTEST"/amend || exit
 	read command sha1 rest < "$TODO"
 	case "$command" in
-	'#'*|'')
+	'#'*|''|noop)
 		mark_action_done
 		;;
 	pick|p)
@@ -584,6 +584,7 @@ first and then run 'git rebase --continue' again."
 			--abbrev=7 --reverse --left-right --cherry-pick \
 			$UPSTREAM...$HEAD | \
 			sed -n "s/^>/pick /p" > "$TODO"
+		test -s "$TODO" || echo noop >> "$TODO"
 		cat >> "$TODO" << EOF
 
 # Rebase $SHORTUPSTREAM..$SHORTHEAD onto $SHORTONTO

t/t3404-rebase-interactive.sh

@@ -419,4 +419,15 @@ test_expect_success 'rebase with a file named HEAD in worktree' '
 
 '
 
+test_expect_success 'do "noop" when there is nothing to cherry-pick' '
+
+	git checkout -b branch4 HEAD &&
+	GIT_EDITOR=: git commit --amend \
+		--author="Somebody else <somebody@else.com>" 
+	test $(git rev-parse branch3) != $(git rev-parse branch4) &&
+	git rebase -i branch3 &&
+	test $(git rev-parse branch3) = $(git rev-parse branch4)
+
+'
+
 test_done