patch 8.2.4860: MS-Windows: always uses current directory for executables

Problem:    MS-Windows: always uses current directory for executables.
Solution:   Check the NoDefaultCurrentDirectoryInExePath environment variable.
            (Yasuhiro Matsumoto, closes #10341)

runtime/doc/builtin.txt

@@ -2016,8 +2016,10 @@ executable({expr})					*executable()*
 		On MS-Windows it only checks if the file exists and is not a
 		directory, not if it's really executable.
 		On MS-Windows an executable in the same directory as Vim is
-		always found.  Since this directory is added to $PATH it
+		normally found.  Since this directory is added to $PATH it
-		should also work to execute it |win32-PATH|.
+		should also work to execute it |win32-PATH|.  This can be
+		disabled by setting the $NoDefaultCurrentDirectoryInExePath
+		environment variable.  *NoDefaultCurrentDirectoryInExePath*
 		The result is a Number:
 			1	exists
 			0	does not exist

src/os_win32.c

@@ -2237,7 +2237,11 @@ executable_exists(char *name, char_u **path, int use_path, int use_pathext)
 		retval = FALSE;
 		goto theend;
 	    }
+
+	    if (mch_getenv("NoDefaultCurrentDirectoryInExePath") == NULL)
 		STRCPY(pathbuf, ".;");
+	    else
+		*pathbuf = NUL;
 	    STRCAT(pathbuf, p);
 	}
     }

src/testdir/test_functions.vim

@@ -2906,4 +2906,22 @@ func Test_isabsolutepath()
   endif
 endfunc
 
+" Test for exepath()
+func Test_exepath()
+  if has('win32')
+    call assert_notequal(exepath('cmd'), '')
+
+    let oldNoDefaultCurrentDirectoryInExePath = $NoDefaultCurrentDirectoryInExePath
+    call writefile(['@echo off', 'echo Evil'], 'vim-test-evil.bat')
+    let $NoDefaultCurrentDirectoryInExePath = ''
+    call assert_notequal(exepath("vim-test-evil.bat"), '')
+    let $NoDefaultCurrentDirectoryInExePath = '1'
+    call assert_equal(exepath("vim-test-evil.bat"), '')
+    let $NoDefaultCurrentDirectoryInExePath = oldNoDefaultCurrentDirectoryInExePath
+    call delete('vim-test-evil.bat')
+  else
+    call assert_notequal(exepath('sh'), '')
+  endif
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4860,
 /**/
     4859,
 /**/