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/colliding-file-names

@@ -0,0 +1,19 @@
+#!/bin/sh
+# Check that diff responds well if a directory has multiple file names
+# that compare equal.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+mkdir d1 d2 || fail=1
+
+for i in abc abC aBc aBC; do
+ echo xyz >d1/$i || fail=1
+done
+
+for i in ABC ABc AbC Abc; do
+ echo xyz >d2/$i || fail=1
+done
+
+diff -r --ignore-file-name-case d1 d2 || fail=1
+
+Exit $fail