patch 8.1.1593: filetype not detected for C++ header files without extension

Problem:    Filetype not detected for C++ header files without extension.
Solution:   Recognize the file by the Emacs file mode. (Dmitry Ilyin,
            closes #4593)

runtime/scripts.vim

@@ -212,6 +212,11 @@ else
   elseif s:line1 =~# '^" *[vV]im$'
     set ft=vim
 
+  " libcxx and libstdc++ standard library headers like "iostream" do not have
+  " an extension, recognize the Emacs file mode.
+  elseif s:line1 =~? '-\*-.*C++.*-\*-'
+    set ft=cpp
+
   " MOO
   elseif s:line1 =~# '^\*\* LambdaMOO Database, Format Version \%([1-3]\>\)\@!\d\+ \*\*$'
     set ft=moo

src/testdir/test_filetype.vim

@@ -586,6 +586,8 @@ let s:script_checks = {
       \ 'cfengine': [['#!/path/cfengine']],
       \ 'erlang': [['#!/path/escript']],
       \ 'haskell': [['#!/path/haskell']],
+      \ 'cpp': [['// Standard iostream objects -*- C++ -*-'],
+      \         ['// -*- C++ -*-']],
       \ }
 
 func Test_script_detection()

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1593,
 /**/
     1592,
 /**/