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.

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