patch 8.0.1820: terminal window redirecting stdout does not show stderr

Problem:    Terminal window redirecting stdout does not show stderr. (Matéo
            Zanibelli)
Solution:   When stdout is not connected to pty_master_fd then use it for
            stderr. (closes #2903)

src/os_unix.c

@@ -5645,7 +5645,12 @@ mch_job_start(char **argv, job_T *job, jobopt_T *options)
 	/* When using pty_master_fd only set it for stdout, do not duplicate it
 	 * for stderr, it only needs to be read once. */
 	int err_fd = use_out_for_err || use_file_for_err || use_null_for_err
-		      ? INVALID_FD : fd_err[0] < 0 ? INVALID_FD : fd_err[0];
+		      ? INVALID_FD
+		      : fd_err[0] >= 0
+		         ? fd_err[0]
+		         : (out_fd == pty_master_fd
+				 ? INVALID_FD
+				 : pty_master_fd);
 
 	channel_set_pipes(channel, in_fd, out_fd, err_fd);
 	channel_set_job(channel, job, options);

src/testdir/test_terminal.vim

@@ -1484,3 +1484,25 @@ func Test_terminal_termwinkey()
   call feedkeys("\<C-L>\<C-C>", 'tx')
   call WaitForAssert({-> assert_equal("dead", job_status(job))})
 endfunc
+
+func Test_terminal_out_err()
+  if !has('unix')
+    return
+  endif
+  call writefile([
+	\ '#!/bin/sh',
+	\ 'echo "this is standard error" >&2',
+	\ 'echo "this is standard out" >&1',
+	\ ], 'Xechoerrout.sh')
+  call setfperm('Xechoerrout.sh', 'rwxrwx---')
+
+  let outfile = 'Xtermstdout'
+  let buf = term_start(['./Xechoerrout.sh'], {'out_io': 'file', 'out_name': outfile})
+  call WaitForAssert({-> assert_inrange(1, 2, len(readfile(outfile)))})
+  call assert_equal("this is standard out", readfile(outfile)[0])
+  call assert_equal('this is standard error', term_getline(buf, 1))
+
+  exe buf . 'bwipe'
+  call delete('Xechoerrout.sh')
+  call delete(outfile)
+endfunc

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1820,
 /**/
     1819,
 /**/