patch 8.0.1708: mkdir with 'p' flag fails on existing directory

Problem:    Mkdir with 'p' flag fails on existing directory, which is
            different from the mkdir shell command.
Solution:   Don't fail if the directory already exists. (James McCoy,
            closes #2775)

src/testdir/test_eval_stuff.vim

@@ -25,3 +25,20 @@ func Test_nocatch_restore_silent_emsg()
   let c5 = nr2char(screenchar(&lines, 5))
   call assert_equal('wrong', c1 . c2 . c3 . c4 . c5)
 endfunc
+
+func Test_mkdir_p()
+  call mkdir('Xmkdir/nested', 'p')
+  call assert_true(isdirectory('Xmkdir/nested'))
+  try
+    " Trying to make existing directories doesn't error
+    call mkdir('Xmkdir', 'p')
+    call mkdir('Xmkdir/nested', 'p')
+  catch /E739:/
+    call assert_report('mkdir(..., "p") failed for an existing directory')
+  endtry
+  " 'p' doesn't suppress real errors
+  call writefile([], 'Xfile')
+  call assert_fails('call mkdir("Xfile", "p")', 'E739')
+  call delete('Xfile')
+  call delete('Xmkdir', 'rf')
+endfunc