patch 8.2.0942: expanding to local dir after homedir keeps "~/"

Problem:    Expanding to local dir after homedir keeps "~/".
Solution:   Adjust modify_fname(). (Christian Brabandt, closes #6205,
            closes #5979)
diff --git a/src/testdir/test_fnamemodify.vim b/src/testdir/test_fnamemodify.vim
index b90d232..f8ce0b1 100644
--- a/src/testdir/test_fnamemodify.vim
+++ b/src/testdir/test_fnamemodify.vim
@@ -31,6 +31,9 @@
   call assert_equal('tar', fnamemodify('abc.fb2.tar.gz', ':e:e:r'))
 
   let cwd = getcwd()
+  call chdir($HOME)
+  call assert_equal('foobar', fnamemodify('~/foobar', ':~:.'))
+  call chdir(cwd)
   call mkdir($HOME . '/XXXXXXXX/a', 'p')
   call mkdir($HOME . '/XXXXXXXX/b', 'p')
   call chdir($HOME . '/XXXXXXXX/a/')