hush: implement negative start in the ${v: -n[:m]} idiom

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/ash_test/ash-vars/var_bash6.right b/shell/ash_test/ash-vars/var_bash6.right
new file mode 100644
index 0000000..63fc23d
--- /dev/null
+++ b/shell/ash_test/ash-vars/var_bash6.right
@@ -0,0 +1,5 @@
+Expected Actual
+a*z    : a*z
+\z     : \z
+a1z a2z: a1z a2z
+z      : z