patch 8.0.1231: expanding file name drops dash

Problem:    Expanding file name drops dash. (stucki)
Solution:   Use the right position. (Christian Brabandt, closes #2184)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index ff19082..832413e 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -365,6 +365,27 @@
   delcommand Foo
 endfunc
 
+func Test_cmdline_write_alternatefile()
+  new
+  call setline('.', ['one', 'two'])
+  f foo.txt
+  new
+  f #-A
+  call assert_equal('foo.txt-A', expand('%'))
+  f #<-B.txt
+  call assert_equal('foo-B.txt', expand('%'))
+  f %<
+  call assert_equal('foo-B', expand('%'))
+  new
+  call assert_fails('f #<', 'E95')
+  bw!
+  f foo-B.txt
+  f %<-A
+  call assert_equal('foo-B-A', expand('%'))
+  bw!
+  bw!
+endfunc
+
 " using a leading backslash here
 set cpo+=C