Improve UX around missing submodules and '--recurse-submodules' for clone and checkout

[phil-blain]

When a user clones a repo with --recurse-submodules, clone writes the pathspec . to the config submodule.active.

If this repo at some point in its history had a submodule, but this submodule was then removed, then checking out an older commit where the submodule is still present fails if the checkout uses --recurse-submodules.

Reproducer:

git init checkout-removed-submodule &&
cd checkout-removed-submodule/ &&
echo "Hello, world" >foo &&
git add foo && git commit -m "Initial commit" &&
git init ../submodule &&
cd ../submodule/ &&
echo "Foo bar" >foobar.txt &&
git add foobar.txt && git commit -m "Foo bar baz" &&
cd ../checkout-removed-submodule/ &&
git submodule add ../submodule/ foobar &&
git commit -m "Add foobar submodule" &&
git rm foobar/ &&
git commit -m "Remove foobar submodule" &&
cd ../ &&
git clone --recurse-submodules checkout-removed-submodule cloned-repo  &&
cd cloned-repo && 
git co --recurse-submodules HEAD~
  fatal: not a git repository: ../.git/modules/foobar
  fatal: could not reset submodule index

Cf:
https://lore.kernel.org/git/[email protected]/T/#u
https://lore.kernel.org/git/CAE5ih78zCR0ZdHAjoxguUb3Y6KFkZcoxJjhS7rkbtZpr+d1n=g@mail.gmail.com/

A lot of improvements could be made here.

[phil-blain]

Also related:

• https://public-inbox.org/git/[email protected]/
• https://lore.kernel.org/git/[email protected]/ (different scenario, checkout with a pathspec)
• https://public-inbox.org/git/CABVQXt5E-R22G62W-tQieA7XiZKZiOA8Hp2xewYhwwOS8wFh0Q@mail.gmail.com/ (slightly different scenario with update=none)
• https://lore.kernel.org/git/DB6PR0101MB2344147D4749598823B094F1D97E0@DB6PR0101MB2344.eurprd01.prod.exchangelabs.com/
• https://lore.kernel.org/git/CWXP265MB3688191D75A56F27A0C4DDB69CFD9@CWXP265MB3688.GBRP265.PROD.OUTLOOK.COM/T/#u