patch 9.0.0896: test for home key fails when 'term' is "tmux"

Problem:    Test for home key fails when 'term' is "tmux".
Solution:   Only save termcap entries that exist.  Adjust code for xHome to
            what xterm uses. (closes #11566)

src/testdir/test_termcodes.vim

@@ -2534,29 +2534,31 @@ endfunc
 func Test_home_key_works()
   " The '@' character in K_HOME must only match "1" when followed by ";",
   " otherwise this code for Home is not recognized: "<Esc>[1~"
-  " Set termcap values like "xterm" uses them.
-  let save_kh = &t_kh
-  let save_K1 = &t_K1
-  let save_k2 = &t_k2
-  let save_k3 = &t_k3
-  let save_end = &t_@7
+  " Set termcap values like "xterm" uses them.  Except using F2 for xHome,
+  " because that termcap entry can't be set here.
+  let save_K1 = exists('&t_K1') ? &t_K1 : ''
+  let save_kh = exists('&t_kh') ? &t_kh : ''
+  let save_k2 = exists('&t_k2') ? &t_k2 : ''
+  let save_k3 = exists('&t_k3') ? &t_k3 : ''
+  let save_end = exists('&t_@7') ? &t_@7 : ''
+
   let &t_K1 = "\<Esc>[1;*~"      " <kHome>
   let &t_kh = "\<Esc>[@;*H"      " <Home>
-  let &t_k2 = "\<Esc>O*H[1;*~"   " use <F2> for <xHome>
+  let &t_k2 = "\<Esc>O*H"        " use <F2> for <xHome>
   let &t_k3 = "\<Esc>[7;*~"      " use <F3> for <zHome>
   let &t_@7 = "\<Esc>[@;*F"      " <End>
 
   new
   call feedkeys("i\<C-K>\<Esc>OH\n\<Esc>", 'tx')
   call feedkeys("i\<C-K>\<Esc>[1~\n\<Esc>", 'tx')
   call assert_equal([
-        \ '<Home>',
+        \ '<F2>',
         \ '<kHome>',
         \ ''], getline(1, '$'))
 
   bwipe!
-  let &t_kh = save_kh
   let &t_K1 = save_K1
+  let &t_kh = save_kh
   let &t_k2 = save_k2
   let &t_k3 = save_k3
   let &t_@7 = save_end

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    896,
 /**/
     895,
 /**/