From: Derrick Stolee <dstolee@microsoft.com>

Prior to 14c7fa269e4 (check_repository_format_gently(): refuse extensions
for old repositories, 2020-06-05), Git was honoring configured
extensions, even if core.repositoryFormatVersion was 0 (or unset). This
was incorrect, and is now fixed.

The issue now is that users who relied on that previously bad behavior
will upgrade to the next version of Git and suddently be in a bad
situation. In particular, users of the 'git sparse-checkout' builting
will rely on the extensions.worktreeConfig for the core.sparseCheckout
and core.sparseCheckoutCone config options. Without that extension,
these users will suddenly have repositories stop acting like a sparse
repo.

What is worse is that a user will be confronted with the following
error if they try to run 'git sparse-checkout init' again:

	warning: unable to upgrade repository format from 0 to 1

This is because the logic in 14c7fa269e4 refuses to upgrae repos when
the version is unset but extensions exist.

While it is important to correct this improper behavior, create a
warning so users in this situation can correct themselves without too
much guesswork. By creating a warning in
check_repository_format_gently(), we can alert the user if they have a
ocnfigured extension but not a configured repository version.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Submitted-As: https://lore.kernel.org/git/pull.674.git.1594668051847.gitgitgadget@gmail.com