cmdline: CTRL-R: <Space> instead of CR between lines.

^M isn't any more "correct" than space: 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 that the user can review the command. We can do
that with a space instead, and then the command can be executed without
having to fix it up first.

test/functional/cmdline/ctrl_r_spec.lua

@@ -0,0 +1,34 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, insert, funcs, eq, feed =
+  helpers.clear, helpers.insert, helpers.funcs, helpers.eq, helpers.feed
+
+describe('cmdline CTRL-R', function()
+  before_each(clear)
+
+  it('pasting non-special register inserts <Space> 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())
+
+    -- 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())
+
+    -- Yank 1 line linewise, then paste to cmdline.
+    feed([[<C-\><C-N>ggyy:<C-R>0]])
+    -- No spaces inserted.
+    eq('line1abc', funcs.getcmdline())
+  end)
+
+  it('pasting special register inserts <CR>, <NL>', function()
+    feed([[:<C-R>="foo\nbar\rbaz"<CR>]])
+    eq('foo\nbar\rbaz', funcs.getcmdline())
+  end)
+end)
+