patch 8.2.4566: check for existing buffer in session file may not work

Problem:    Check for existing buffer in session file does not work for files
            in the home directory.
Solution:   Use fnamemodify(). (James Cherti, closes #9945)  Add a test.
diff --git a/src/testdir/test_mksession.vim b/src/testdir/test_mksession.vim
index 494ac87..2c2737e 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -245,6 +245,7 @@
   let count1 = 0
   let count2 = 0
   let count2buf = 0
+  let bufexists = 0
   for line in lines
     if line =~ 'edit \f*Xtest1$'
       let count1 += 1
@@ -255,10 +256,14 @@
     if line =~ 'buffer \f\{-}Xtest2'
       let count2buf += 1
     endif
+    if line =~ 'bufexists(fnamemodify(.*, ":p")'
+      let bufexists += 1
+    endif
   endfor
   call assert_equal(1, count1, 'Xtest1 count')
   call assert_equal(2, count2, 'Xtest2 count')
   call assert_equal(2, count2buf, 'Xtest2 buffer count')
+  call assert_equal(2, bufexists)
 
   close
   bwipe!