vim-patch:8.0.1053: setline() does not work on startup

Problem:    setline() does not work on startup. (Manuel Ortega)
Solution:   Do not check for ml_mfp to be set for the current buffer.
            (Christian Brabandt)
9d954207e2

src/nvim/eval.c

@@ -14854,7 +14854,10 @@ static void set_buffer_lines(buf_T *buf, linenr_T lnum, typval_T *lines,
   buf_T       *curbuf_save;
   int         is_curbuf = buf == curbuf;
 
-  if (buf == NULL || buf->b_ml.ml_mfp == NULL || lnum < 1) {
+  // When using the current buffer ml_mfp will be set if needed.  Useful when
+  // setline() is used on startup.  For other buffers the buffer must be
+  // loaded.
+  if (buf == NULL || (!is_curbuf && buf->b_ml.ml_mfp == NULL) || lnum < 1) {
     rettv->vval.v_number = 1;  // FAIL
     return;
   }

src/nvim/testdir/shared.vim

@@ -190,12 +190,18 @@ func s:feedkeys(timer)
 endfunc
 
 " Get the command to run Vim, with -u NONE and --headless arguments.
+" If there is an argument use it instead of "NONE".
 " Returns an empty string on error.
-func GetVimCommand()
+func GetVimCommand(...)
+  if a:0 == 0
+    let name = 'NONE'
+  else
+    let name = a:1
+  endif
   let cmd = v:progpath
-  let cmd = substitute(cmd, '-u \f\+', '-u NONE', '')
+  let cmd = substitute(cmd, '-u \f\+', '-u ' . name, '')
-  if cmd !~ '-u NONE'
+  if cmd !~ '-u '. name
-    let cmd = cmd . ' -u NONE'
+    let cmd = cmd . ' -u ' . name
   endif
   let cmd .= ' --headless -i NONE'
   let cmd = substitute(cmd, 'VIMRUNTIME=.*VIMRUNTIME;', '', '')

src/nvim/testdir/test_bufline.vim

@@ -1,5 +1,7 @@
 " Tests for setbufline() and getbufline()
 
+source shared.vim
+
 func Test_setbufline_getbufline()
   new
   let b = bufnr('%')
@@ -24,3 +26,16 @@ func Test_setbufline_getbufline()
   call assert_equal([], getbufline(b, 6))
   exe "bwipe! " . b
 endfunc
+
+func Test_setline_startup()
+  let cmd = GetVimCommand('Xscript')
+  if cmd == ''
+    return
+  endif
+  call writefile(['call setline(1, "Hello")', 'w Xtest', 'q!'], 'Xscript')
+  call system(cmd)
+  call assert_equal(['Hello'], readfile('Xtest'))
+
+  call delete('Xscript')
+  call delete('Xtest')
+endfunc