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.

src/session.c

@@ -385,9 +385,9 @@ put_view(
 	    // Note, if a buffer for that file already exists, use :badd to
 	    // edit that buffer, to not lose folding information (:edit resets
 	    // folds in other buffers)
-	    if (fputs("if bufexists(\"", fd) < 0
+	    if (fputs("if bufexists(fnamemodify(\"", fd) < 0
 		    || ses_fname(fd, wp->w_buffer, flagp, FALSE) == FAIL
-		    || fputs("\") | buffer ", fd) < 0
+		    || fputs("\", \":p\")) | buffer ", fd) < 0
 		    || ses_fname(fd, wp->w_buffer, flagp, FALSE) == FAIL
 		    || fputs(" | else | edit ", fd) < 0
 		    || ses_fname(fd, wp->w_buffer, flagp, FALSE) == FAIL

src/testdir/test_mksession.vim

@@ -245,6 +245,7 @@ func Test_mksession_one_buffer_two_windows()
   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 @@ func Test_mksession_one_buffer_two_windows()
     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!

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4566,
 /**/
     4565,
 /**/