refactor(change): do API changes to buffer without curbuf switch

Most of the messy things when changing a non-current buffer is
not about the buffer, it is about windows. In particular, it is about
`curwin`.

When editing a non-current buffer which is displayed in some other
window in the current tabpage, one such window will be "borrowed" as the
curwin. But this means if two or more non-current windows displayed the buffers,
one of them will be treated differenty. this is not desirable.

In particular, with nvim_buf_set_text, cursor _column_ position was only
corrected for one single window. Two new tests are added: the test
with just one non-current window passes, but the one with two didn't.

Two corresponding such tests were also added for nvim_buf_set_lines.
This already worked correctly on master, but make sure this is
well-tested for future refactors.

Also, nvim_create_buf no longer invokes autocmds just because you happened
to use `scratch=true`. No option value was changed, therefore OptionSet
must not be fired.

test/functional/api/vim_spec.lua

@@ -28,7 +28,6 @@ local write_file = helpers.write_file
 local exec_lua = helpers.exec_lua
 local exc_exec = helpers.exc_exec
 local insert = helpers.insert
-local expect_exit = helpers.expect_exit
 local skip = helpers.skip
 
 local pcall_err = helpers.pcall_err
@@ -2832,7 +2831,9 @@ describe('API', function()
 
     it('does not cause heap-use-after-free on exit while setting options', function()
       command('au OptionSet * q')
-      expect_exit(command, 'silent! call nvim_create_buf(0, 1)')
+      command('silent! call nvim_create_buf(0, 1)')
+      -- nowadays this works because we don't execute any spurious autocmds at all #24824
+      assert_alive()
     end)
   end)