cmdline: CTRL-R: Omit trailing <CR>.

The "technically correct" interpretation is to execute the first line
that is seen (and this is what happens on middle-click paste in Vim).
^M is only intended to "defuse" the newline, so the user can review it.

The parent commit changed the behavior to insert <Space> between lines,
but that's a higher-risk change: it is arguably possible that some user
*wants* the literal ^M chars when e.g. assigning to a register:
    :let @a='<C-R>b'

To avoid that risk, keep the old behavior and only omit the last ^M.
This makes `yy:<C-R>0` nicer at no cost.

test/functional/cmdline/ctrl_r_spec.lua

@@ -5,24 +5,24 @@ local clear, insert, funcs, eq, feed =
 describe('cmdline CTRL-R', function()
   before_each(clear)
 
-  it('pasting non-special register inserts <Space> between lines', function()
+  it('pasting non-special register inserts <CR> *between* lines', function()
     insert([[
     line1abc
     line2somemoretext
     ]])
     -- Yank 2 lines linewise, then paste to cmdline.
     feed([[<C-\><C-N>gg0yj:<C-R>0]])
-    -- <Space> inserted *between* lines, not after the final line.
-    eq('line1abc line2somemoretext', funcs.getcmdline())
+    -- <CR> inserted between lines, NOT after the final line.
+    eq('line1abc\rline2somemoretext', funcs.getcmdline())
 
     -- Yank 2 lines characterwise, then paste to cmdline.
     feed([[<C-\><C-N>gg05lyvj:<C-R>0]])
-    -- <Space> inserted *between* lines, not after the final line.
-    eq('abc line2', funcs.getcmdline())
+    -- <CR> inserted between lines, NOT after the final line.
+    eq('abc\rline2', funcs.getcmdline())
 
     -- Yank 1 line linewise, then paste to cmdline.
     feed([[<C-\><C-N>ggyy:<C-R>0]])
-    -- No spaces inserted.
+    -- No <CR> inserted.
     eq('line1abc', funcs.getcmdline())
   end)