api: Implement vim_command_output function

This function can be used by API clients to execute a command and capture the
output.

src/nvim/api/vim.c

@@ -96,6 +96,19 @@ String vim_replace_termcodes(String str, Boolean from_part, Boolean do_lt,
   return cstr_as_string(ptr);
 }
 
+String vim_command_output(String str, Error *err)
+{
+  do_cmdline_cmd((char_u *)"redir => v:command_output");
+  vim_command(str, err);
+  do_cmdline_cmd((char_u *)"redir END");
+
+  if (err->set) {
+    return (String) STRING_INIT;
+  }
+
+  return cstr_to_string((char *)get_vim_var_str(VV_COMMAND_OUTPUT));
+}
+
 /// Evaluates the expression str using the vim internal expression
 /// evaluator (see |expression|).
 /// Dictionaries and lists are recursively expanded.

src/nvim/eval.c

@@ -424,7 +424,8 @@ static struct vimvar {
   {VV_NAME("oldfiles",         VAR_LIST), 0},
   {VV_NAME("windowid",         VAR_NUMBER), VV_RO},
   {VV_NAME("progpath",         VAR_STRING), VV_RO},
-  {VV_NAME("job_data",         VAR_LIST), 0}
+  {VV_NAME("job_data",         VAR_LIST), 0},
+  {VV_NAME("command_output",   VAR_STRING), 0}
 };
 
 /* shorthand */

src/nvim/eval.h

@@ -64,6 +64,7 @@ enum {
     VV_WINDOWID,
     VV_PROGPATH,
     VV_JOB_DATA,
+    VV_COMMAND_OUTPUT,
     VV_LEN, /* number of v: vars */
 };