stash: optimize `get_untracked_files()` and `check_changes()`

stash: optimize `get_untracked_files()` and `check_changes()`

This commits introduces a optimization by avoiding calling the
same functions again. For example, `git stash push -u`
would call at some points the following functions:

 * `check_changes()` (inside `do_push_stash()`)
 * `do_create_stash()`, which calls: `check_changes()` and
`get_untracked_files()`

Note that `check_changes()` also calls `get_untracked_files()`.
So, `check_changes()` is called 2 times and `get_untracked_files()`
3 times.

The old function `check_changes()` now consists of two functions:
`get_untracked_files()` and `check_changes_tracked_files()`.

These are the call chains for `push` and `create`:

 * `push_stash()` -> `do_push_stash()` -> `do_create_stash()`

 * `create_stash()` -> `do_create_stash()`

To prevent calling the same functions over and over again,
`check_changes()` inside `do_create_stash()` is now placed
in the caller functions (`create_stash()` and `do_push_stash()`).
This way `check_changes()` and `get_untracked files()` are called
only one time.

Signed-off-by: Paul-Sebastian Ungureanu <ungureanupaulsebastian@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>