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.

runtime/doc/vim_diff.txt

@@ -238,8 +238,7 @@ newly allocated memory all over the place) and fail on types which cannot be
 coerced to strings. See |id()| for more details, currently it uses 
 `printf("%p", {expr})` internally.
 
-|c_CTRL-R| pasting a non-special register into the |cmdline| separates lines
-by <Space> instead of <CR>.
+|c_CTRL-R| pasting a non-special register into |cmdline| omits the last <CR>.
 
 ==============================================================================
 5. Missing legacy features				 *nvim-features-missing*

src/nvim/ex_getln.c

@@ -2471,10 +2471,10 @@ void restore_cmdline_alloc(char_u *p)
 ///
 /// @param regname   Register name.
 /// @param literally Insert text literally instead of "as typed".
-/// @param remspc    When true, remove trailing <Space>.
+/// @param remcr     When true, remove trailing CR.
 ///
 /// @returns FAIL for failure, OK otherwise
-static bool cmdline_paste(int regname, bool literally, bool remspc)
+static bool cmdline_paste(int regname, bool literally, bool remcr)
 {
   long i;
   char_u              *arg;
@@ -2539,7 +2539,7 @@ static bool cmdline_paste(int regname, bool literally, bool remspc)
     return OK;
   }
 
-  return cmdline_paste_reg(regname, literally, remspc);
+  return cmdline_paste_reg(regname, literally, remcr);
 }
 
 /*

src/nvim/ops.c

@@ -1266,10 +1266,10 @@ int get_spec_reg(
 ///
 /// @param regname   Register name.
 /// @param literally Insert text literally instead of "as typed".
-/// @param remspc    When true, don't add <Space> characters.
+/// @param remcr     When true, don't add CR characters.
 ///
 /// @returns FAIL for failure, OK otherwise
-bool cmdline_paste_reg(int regname, bool literally, bool remspc)
+bool cmdline_paste_reg(int regname, bool literally, bool remcr)
 {
   yankreg_T *reg = get_yank_register(regname, YREG_PASTE);
   if (reg->y_array == NULL)
@@ -1278,9 +1278,9 @@ bool cmdline_paste_reg(int regname, bool literally, bool remspc)
   for (size_t i = 0; i < reg->y_size; i++) {
     cmdline_paste_str(reg->y_array[i], literally);
 
-    // Insert space between lines, unless `remspc` is true.
-    if (i < reg->y_size - 1 && !remspc) {
-      cmdline_paste_str((char_u *)" ", literally);
+    // Insert ^M between lines, unless `remcr` is true.
+    if (i < reg->y_size - 1 && !remcr) {
+      cmdline_paste_str((char_u *)"\r", literally);
     }
 
     /* Check for CTRL-C, in case someone tries to paste a few thousand

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)