shell/logging: Fix E730 with verbose system({List}) #9009

ref https://github.com/neovim/neovim/issues/9001#issuecomment-421843790

Steps to reproduce:
    :set verbose=9
    :call system(['echo'])
    E730: using List as a String

src/nvim/eval.c

@@ -16471,13 +16471,12 @@ static void get_system_output_as_rettv(typval_T *argvars, typval_T *rettv,
   }
 
   if (p_verbose > 3) {
-    char buf[NUMBUFLEN];
-    const char * cmd = tv_get_string_buf(argvars, buf);
-
+    char *cmdstr = shell_argv_to_str(argv);
     verbose_enter_scroll();
-    smsg(_("Calling shell to execute: \"%s\""), cmd);
+    smsg(_("Executing command: \"%s\""), cmdstr);
     msg_puts("\n\n");
     verbose_leave_scroll();
+    xfree(cmdstr);
   }
 
   if (do_profiling == PROF_YES) {

src/nvim/eval/typval.c

@@ -2632,7 +2632,7 @@ static const char *const str_errors[] = {
 
 #undef FUNC_ERROR
 
-/// Check that given value is a string or can be converted to it
+/// Check that given value is a VimL String or can be "cast" to it.
 ///
 /// Error messages are compatible with tv_get_string_chk() previously used for
 /// the same purpose.
@@ -2805,7 +2805,7 @@ float_T tv_get_float(const typval_T *const tv)
   return 0;
 }
 
-/// Get the string value of a VimL object
+/// Get the string value of a "stringish" VimL object.
 ///
 /// @param[in]  tv  Object to get value of.
 /// @param  buf  Buffer used to hold numbers and special variables converted to
@@ -2847,7 +2847,7 @@ const char *tv_get_string_buf_chk(const typval_T *const tv, char *const buf)
   return NULL;
 }
 
-/// Get the string value of a VimL object
+/// Get the string value of a "stringish" VimL object.
 ///
 /// @warning For number and special values it uses a single, static buffer. It
 ///          may be used only once, next call to get_tv_string may reuse it. Use
@@ -2866,7 +2866,7 @@ const char *tv_get_string_chk(const typval_T *const tv)
   return tv_get_string_buf_chk(tv, mybuf);
 }
 
-/// Get the string value of a VimL object
+/// Get the string value of a "stringish" VimL object.
 ///
 /// @warning For number and special values it uses a single, static buffer. It
 ///          may be used only once, next call to get_tv_string may reuse it. Use
@@ -2888,7 +2888,7 @@ const char *tv_get_string(const typval_T *const tv)
   return tv_get_string_buf((typval_T *)tv, mybuf);
 }
 
-/// Get the string value of a VimL object
+/// Get the string value of a "stringish" VimL object.
 ///
 /// @note tv_get_string_chk() and tv_get_string_buf_chk() are similar, but
 ///       return NULL on error.

src/nvim/misc1.c

@@ -2718,7 +2718,7 @@ int call_shell(char_u *cmd, ShellOpts opts, char_u *extra_shell_arg)
 
   if (p_verbose > 3) {
     verbose_enter();
-    smsg(_("Calling shell to execute: \"%s\""), cmd == NULL ? p_sh : cmd);
+    smsg(_("Executing command: \"%s\""), cmd == NULL ? p_sh : cmd);
     msg_putchar('\n');
     verbose_leave();
   }

src/nvim/os/shell.c

@@ -80,21 +80,53 @@ char **shell_build_argv(const char *cmd, const char *extra_args)
 void shell_free_argv(char **argv)
 {
   char **p = argv;
-
   if (p == NULL) {
     // Nothing was allocated, return
     return;
   }
-
   while (*p != NULL) {
     // Free each argument
     xfree(*p);
     p++;
   }
-
   xfree(argv);
 }
 
+/// Joins shell arguments from `argv` into a new string.
+/// If the result is too long it is truncated with ellipsis ("...").
+///
+/// @returns[allocated] `argv` joined to a string.
+char *shell_argv_to_str(char **const argv)
+  FUNC_ATTR_NONNULL_ALL
+{
+  size_t n = 0;
+  char **p = argv;
+  char *rv = xcalloc(256, sizeof(*rv));
+  const size_t maxsize = (256 * sizeof(*rv));
+  if (*p == NULL) {
+    return rv;
+  }
+  while (*p != NULL) {
+    xstrlcat(rv, "'", maxsize);
+    xstrlcat(rv, *p, maxsize);
+    n = xstrlcat(rv,  "' ", maxsize);
+    if (n >= maxsize) {
+      break;
+    }
+    p++;
+  }
+  if (n < maxsize) {
+    rv[n - 1] = '\0';
+  } else {
+    // Command too long, show ellipsis: "/bin/bash 'foo' 'bar'..."
+    rv[maxsize - 4] = '.';
+    rv[maxsize - 3] = '.';
+    rv[maxsize - 2] = '.';
+    rv[maxsize - 1] = '\0';
+  }
+  return rv;
+}
+
 /// Calls the user-configured 'shell' (p_sh) for running a command or wildcard
 /// expansion.
 ///