MacOs and Windows lowercases filenames

[pared]

Context: https://discordapp.com/channels/485586884165107732/485596304961962003/677178126916124702
The issue:
User created data dir which contained files of the same name, some were uppercase, some were lowercase.
Other user cloned the repository on MacOs and could not get images containing uppercase letters,
when those names ovelapped with other images having lowercase (the same) name.
Pulling on linux was fine.

Reproduction script:

#!/bin/bash

rm -rf repo storage git_repo erepo
mkdir erepo git_repo storage

MAIN=$(pwd)

pushd git_repo
git init --bare --quiet
popd
set -ex
pushd erepo
git init --quiet
dvc init -q

mkdir data_dir
echo data1>>data_dir/file
echo data2>>data_dir/File

dvc add data_dir
dvc remote add -d str $MAIN/storage
git add -A
git commit -am "initial"
git remote add origin $MAIN/git_repo

dvc push
git push origin master
popd

git clone git_repo repo

pushd repo
dvc pull

after pull, we should have file and File in target repo, which seems to not happen on MacOs.

[pared]

So, turns out that by default, Windows and MacOs do not support case sensitive file names.
Some resources:
https://superuser.com/questions/165975/are-all-versions-of-windows-case-insensitive
https://apple.stackexchange.com/questions/22297/is-bash-in-osx-case-insensitive

It seems that this filesystem setting, so one should be able to configure case sensitive support.
It doesn't seem to me that we can do something about it at dvc level.

[pared]

We could probably warn the user about this behavior, though the detection of such use cases could be resource consuming.