vim-patch:8.2.3363: when :edit reuses the current buffer the alternate file is set (#19306)

Problem:    When :edit reuses the current buffer the alternate file is set to
            the same buffer.
Solution:   Only set the alternate file when not reusing the buffer.
            (closes vim/vim#8783)
b8bd2e6eba

Cherry-pick Test_cmdline_expand_special() from patches 8.2.{0243,2873}.
Move Test_cmd_backtick() to the right place.

src/nvim/testdir/test_cmdline.vim

@@ -1328,14 +1328,6 @@ func Test_cmdwin_jump_to_win()
   call assert_equal(1, winnr('$'))
 endfunc
 
-" Test for backtick expression in the command line
-func Test_cmd_backtick()
-  %argd
-  argadd `=['a', 'b', 'c']`
-  call assert_equal(['a', 'b', 'c'], argv())
-  %argd
-endfunc
-
 func Test_cmdlineclear_tabenter()
   " See test/functional/legacy/cmdline_spec.lua
   CheckScreendump
@@ -1355,6 +1347,32 @@ func Test_cmdlineclear_tabenter()
   call delete('XtestCmdlineClearTabenter')
 endfunc
 
+" Test for expanding special keywords in cmdline
+func Test_cmdline_expand_special()
+  new
+  %bwipe!
+  call assert_fails('e #', 'E194:')
+  call assert_fails('e <afile>', 'E495:')
+  call assert_fails('e <abuf>', 'E496:')
+  call assert_fails('e <amatch>', 'E497:')
+  call writefile([], 'Xfile.cpp')
+  call writefile([], 'Xfile.java')
+  new Xfile.cpp
+  call feedkeys(":e %:r\<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_equal('"e Xfile.cpp Xfile.java', @:)
+  close
+  call delete('Xfile.cpp')
+  call delete('Xfile.java')
+endfunc
+
+" Test for backtick expression in the command line
+func Test_cmd_backtick()
+  %argd
+  argadd `=['a', 'b', 'c']`
+  call assert_equal(['a', 'b', 'c'], argv())
+  %argd
+endfunc
+
 func Test_cmdwin_tabpage()
   tabedit
   " v8.2.1919 isn't ported yet, so E492 is thrown after E11 here.

src/nvim/testdir/test_undo.vim

@@ -579,7 +579,7 @@ func Test_undofile_2()
 
   " add 10 lines, delete 6 lines, undo 3
   set undofile
-  call setbufline(0, 1, ['one', 'two', 'three', 'four', 'five', 'six',
+  call setbufline('%', 1, ['one', 'two', 'three', 'four', 'five', 'six',
 	      \ 'seven', 'eight', 'nine', 'ten'])
   set undolevels=100
   normal 3Gdd