startup: fix -E/-Es without -u NONE

Before this change, -E/-Es without `-u NONE` reads stdin as Ex commands.
It should always read stdin as text (into buffer 1), like this:

    echo foo | nvim -Es +'%p'
    foo
    echo foo | nvim -Es -u NORC +'%p'
    foo

src/nvim/main.c

@@ -301,9 +301,11 @@ int main(int argc, char **argv)
   // Read ex-commands if invoked with "-es".
   //
   bool reading_tty = !headless_mode
+                     && !silent_mode
                      && (params.input_isatty || params.output_isatty
                          || params.err_isatty);
-  bool reading_excmds = !params.input_isatty && silent_mode
+  bool reading_excmds = !params.input_isatty
+                        && silent_mode
                         && exmode_active == EXMODE_NORMAL;
   if (reading_tty || reading_excmds) {
     // One of the startup commands (arguments, sourced scripts or plugins) may

test/functional/core/startup_spec.lua

@@ -157,6 +157,14 @@ describe('startup', function()
     eq(inputstr,
        funcs.system({nvim_prog, '-i', 'NONE', '-Es', '+%print', '-' },
                     input))
+    -- with `-u NORC`
+    eq('thepartycontinues\n',
+       funcs.system({nvim_prog, '-n', '-u', 'NORC', '-Es', '+.print' },
+                    { 'thepartycontinues', '' }))
+    -- without `-u`
+    eq('thepartycontinues\n',
+       funcs.system({nvim_prog, '-n', '-Es', '+.print' },
+                    { 'thepartycontinues', '' }))
 
     --
     -- -es: read stdin as ex-commands
@@ -167,6 +175,14 @@ describe('startup', function()
     eq('line1\nline2\n',
        funcs.system({nvim_prog, '-i', 'NONE', '-es', '-' },
                     input))
+    -- with `-u NORC`
+    eq('  encoding=utf-8\n',
+       funcs.system({nvim_prog, '-n', '-u', 'NORC', '-es' },
+                    { 'set encoding', '' }))
+    -- without `-u`
+    eq('  encoding=utf-8\n',
+       funcs.system({nvim_prog, '-n', '-es' },
+                    { 'set encoding', '' }))
   end)
 end)