D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

local/recipes/tools/diffutils/source/tests/new-file

@@ -0,0 +1,41 @@
+#!/bin/sh
+# Test --new-file (-N) and --unidirectional-new-file.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+
+echo a > a || fail=1
+
+echo '0a1
+> a' > exp || fail=1
+
+returns_ 1 diff -N - a <&- > out || fail=1
+compare exp out || fail=1
+
+returns_ 1 diff --unidirectional-new-file - a <&- > out || fail=1
+compare exp out || fail=1
+
+returns_ 1 diff -N b - < a > out || fail=1
+compare exp out || fail=1
+
+returns_ 1 diff --unidirectional-new-file b - < a > out || fail=1
+compare exp out || fail=1
+
+echo '1d0
+< a' > exp || fail=1
+
+returns_ 1 diff -N a - <&- > out || fail=1
+compare exp out || fail=1
+
+# With closed standard input, require an exit status of 2
+# and empty stdout.
+returns_ 2 diff --unidirectional-new-file a - <&- > out || fail=1
+compare /dev/null out || fail=1
+
+returns_ 1 diff -N - b < a > out || fail=1
+compare exp out || fail=1
+
+returns_ 2 diff --unidirectional-new-file - b < a > out || fail=1
+
+Exit $fail