patch 9.1.0569: fnamemodify() treats ".." and "../" differently

Problem:  fnamemodify() treats ".." and "../" differently.
Solution: Expand ".." properly like how "/.." is treated in 8.2.3388.
          (zeertzjq)

closes: #15218

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_findfile.vim b/src/testdir/test_findfile.vim
index 9247a81..a5e18b9 100644
--- a/src/testdir/test_findfile.vim
+++ b/src/testdir/test_findfile.vim
@@ -107,6 +107,9 @@
   let l = findfile('bar', ';../', -1)
   call assert_equal(1, len(l))
   call assert_match('.*/Xfinddir1/Xdir2/Xdir3/bar', l[0])
+  let l = findfile('bar', ';..', -1)
+  call assert_equal(1, len(l))
+  call assert_match('.*/Xfinddir1/Xdir2/Xdir3/bar', l[0])
 
   let l = findfile('bar', ';' . save_dir . '/Xfinddir1/Xdir2/', -1)
   call assert_equal(1, len(l))
@@ -117,6 +120,9 @@
   let l = findfile('bar', ';../../', -1)
   call assert_equal(1, len(l))
   call assert_match('.*/Xfinddir1/Xdir2/Xdir3/bar', l[0])
+  let l = findfile('bar', ';../..', -1)
+  call assert_equal(1, len(l))
+  call assert_match('.*/Xfinddir1/Xdir2/Xdir3/bar', l[0])
 
   let l = findfile('bar', ';' . save_dir . '/Xfinddir1/', -1)
   call assert_equal(2, len(l))
@@ -130,6 +136,10 @@
   call assert_equal(2, len(l))
   call assert_match('.*/Xfinddir1/Xdir2/Xdir3/bar', l[0])
   call assert_match('.*/Xfinddir1/bar',             l[1])
+  let l = findfile('bar', ';../../..', -1)
+  call assert_equal(2, len(l))
+  call assert_match('.*/Xfinddir1/Xdir2/Xdir3/bar', l[0])
+  call assert_match('.*/Xfinddir1/bar',             l[1])
 
   " Test combined downwards and upwards search from Xdir2/.
   cd ../..