patch 8.1.1783: MS-Windows: compiler test may fail when using %:S

Problem:    MS-Windows: compiler test may fail when using %:S.
Solution:   Reset 'shellslash'.

src/testdir/test_compiler.vim

@@ -10,6 +10,10 @@ func Test_compiler()
     unlet $LANG
   endif
 
+  " %:S does not work properly with 'shellslash' set
+  let save_shellslash = &shellslash
+  set noshellslash
+
   e Xfoo.pl
   compiler perl
   call assert_equal('perl', b:current_compiler)
@@ -27,6 +31,7 @@ func Test_compiler()
   call assert_match("\n 1 Xfoo.pl:3: Global symbol \"\$foo\" "
   \ .               "requires explicit package name", a)
 
+  let &shellslash = save_shellslash
   call delete('Xfoo.pl')
   bw!
 endfunc

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1783,
 /**/
     1782,
 /**/