patch 8.0.0151: passing buffer content to system() is clumsy

Problem:    To pass buffer content to system() and systemlist() one has to
            first create a string or list.
Solution:   Allow passing a buffer number. (LemonBoy, closes #1240)
diff --git a/src/testdir/test_system.vim b/src/testdir/test_system.vim
new file mode 100644
index 0000000..5e8d68b
--- /dev/null
+++ b/src/testdir/test_system.vim
@@ -0,0 +1,20 @@
+" Tests for system() and systemlist()
+
+function! Test_System()
+  if !executable('echo') || !executable('cat') || !executable('wc')
+    return
+  endif
+  call assert_equal("123\n", system('echo 123'))
+  call assert_equal(['123'], systemlist('echo 123'))
+  call assert_equal('123',   system('cat', '123'))
+  call assert_equal(['123'], systemlist('cat', '123'))
+  call assert_equal(["as\<NL>df"], systemlist('cat', ["as\<NL>df"]))
+  new Xdummy
+  call setline(1, ['asdf', "pw\<NL>er", 'xxxx'])
+  call assert_equal("3\n",  system('wc -l', bufnr('%')))
+  call assert_equal(['3'],  systemlist('wc -l', bufnr('%')))
+  call assert_equal(['asdf', "pw\<NL>er", 'xxxx'],  systemlist('cat', bufnr('%')))
+  bwipe!
+
+  call assert_fails('call system("wc -l", 99999)', 'E86:')
+endfunction