`Makefile` should ask for confirmation before changing `../cpython`

I ran make all while already having an existing clone of cpython, and even though make detected that, it proceeded to alter it without asking for confirmation:

$ make all
...
git clone --depth 1 --no-single-branch https://github.com/python/cpython.git ../cpython/  || echo "cpython exists"
fatal: destination path '../cpython' already exists and is not an empty directory.
cpython exists
cd ../cpython/ && git checkout 3.10
Switched to branch '3.10'
Your branch is behind 'upstream/3.10' by 339 commits, and can be fast-forwarded.
  (use "git pull" to update your local branch)
mkdir -p ../cpython//Doc/locales/zh_TW/LC_MESSAGES
for dirname in $(find . -name '*.po' | xargs -n1 dirname | sort -u | grep -v '^\.$'); do mkdir -p ../cpython//Doc/locales/zh_TW/LC_MESSAGES/$dirname; done
for file in *.po */*.po; do ln -f $file ../cpython//Doc/locales/zh_TW/LC_MESSAGES/$file; done
. ~/.venvs/python-docs-i18n//bin/activate; make -C ../cpython//Doc/ SPHINXOPTS='-j4 -D locale_dirs=locales -D language=zh_TW -D gettext_compact=0' autobuild-dev-html
...

Since the cpython clone already had a 3.10 branch, it switched to it and proceeded to copy files even if the branch was outdated.

It eventually failed while trying to build the docs because of a buggy.rst test file I had in the cpython repo:

checking consistency... failed

Warning, treated as error:
../cpython/Doc/buggy.rst:document isn't included in any toctree
make[2]: *** [Makefile:51: build] Error 2
make[2]: Leaving directory '../cpython/Doc'
make[1]: *** [Makefile:231: autobuild-dev-html] Error 2
make[1]: Leaving directory '../cpython/Doc'
make: *** [Makefile:54: all] Error 2

It would be better if the Makefile asked for confirmation before altering an existing clone of cpython, however it would also be useful to distinguish between an existing clone that was there before the first execution, and an existing clone that was there because make all previously created it.

The Makefile should do the following:

• If the cpython clone doesn't exist, it could create it without asking for confirmation.
• If it exists, it could check if Doc/locales/zh_TW/LC_MESSAGES/ already exists and if the current branch is $(BRANCH) (currently 3.10):
  • if they are, it can assume the clone was previously created by make all and continue without asking for confirmation;
  • if they aren't, it either means that the clone already existed, or that it was altered after the creation, and should ask for confirmation before switching branch and creating files;

In addition the Makefile created the venv in ~/.venvs, altering yet another "external" directory. It would be better if it created a .venv dir in the cwd instead. (I can create a separate issue for this.)