API/nvim_command_output: handle :echon capture (#8265)

ref https://github.com/neovim/python-client/pull/290

src/nvim/api/vim.c

@@ -239,15 +239,17 @@ String nvim_command_output(String command, Error *err)
   }
 
   if (capture_local.ga_len > 1) {
-    // redir always(?) prepends a newline; remove it.
+    String s = (String){
-    char *s = capture_local.ga_data;
+      .data = capture_local.ga_data,
-    assert(s[0] == '\n');
+      .size = (size_t)capture_local.ga_len,
-    memmove(s, s + 1, (size_t)capture_local.ga_len);
-    s[capture_local.ga_len - 1] = '\0';
-    return (String) {  // Caller will free the memory.
-      .data = s,
-      .size = (size_t)(capture_local.ga_len - 1),
     };
+    // redir usually (except :echon) prepends a newline.
+    if (s.data[0] == '\n') {
+      memmove(s.data, s.data + 1, s.size);
+      s.data[s.size - 1] = '\0';
+      s.size = s.size - 1;
+    }
+    return s;  // Caller will free the memory.
   }
 
 theend:

test/functional/api/vim_spec.lua

@@ -73,6 +73,8 @@ describe('api', function()
     it('captures command output', function()
       eq('this is\nspinal tap',
          nvim('command_output', [[echo "this is\nspinal tap"]]))
+      eq('no line ending!',
+         nvim('command_output', [[echon "no line ending!"]]))
     end)
 
     it('captures empty command output', function()