patch 8.0.0625: shellescape() always escapes a newline

Problem:    shellescape() always escapes a newline, which does not work with
            some shells. (Harm te Hennepe)
Solution:   Only escape a newline when the "special" argument is non-zero.
            (Christian Brabandt, closes #1590)
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index e569ef1..f0f656a 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -784,3 +784,28 @@
   delfunc Operator
   delfunc Apply
 endfunc
+
+func Test_shellescape()
+  let save_shell = &shell
+  set shell=bash
+  call assert_equal("'text'", shellescape('text'))
+  call assert_equal("'te\"xt'", shellescape('te"xt'))
+  call assert_equal("'te'\\''xt'", shellescape("te'xt"))
+
+  call assert_equal("'te%xt'", shellescape("te%xt"))
+  call assert_equal("'te\\%xt'", shellescape("te%xt", 1))
+  call assert_equal("'te#xt'", shellescape("te#xt"))
+  call assert_equal("'te\\#xt'", shellescape("te#xt", 1))
+  call assert_equal("'te!xt'", shellescape("te!xt"))
+  call assert_equal("'te\\!xt'", shellescape("te!xt", 1))
+
+  call assert_equal("'te\nxt'", shellescape("te\nxt"))
+  call assert_equal("'te\\\nxt'", shellescape("te\nxt", 1))
+  set shell=tcsh
+  call assert_equal("'te\\!xt'", shellescape("te!xt"))
+  call assert_equal("'te\\\\!xt'", shellescape("te!xt", 1))
+  call assert_equal("'te\\\nxt'", shellescape("te\nxt"))
+  call assert_equal("'te\\\\\nxt'", shellescape("te\nxt", 1))
+
+  let &shell = save_shell
+endfunc