sparse-checkout: make set subsume init

As described at [1], the split of init and set subcommands in sparse-checkout causes multiple issues:

* Poor performance (deleting all tracked files, then later restoring many and maybe even most of them)
* Poor UI (multiple progress bars in wrappers that hide both commands under 1 user-facing command)
* Loss of ignored files under directories the user wanted to keep

This series fixes this bug by providing a single command to switch to a
sparse-checkout: set. It does so by making set able to do the combined work of init and set. It keeps init as-is to give folks time to adapt, but marks it as deprecated. It also makes reapply able to toggle cone/non-cone mode and sparse-index/non-sparse-index mode.

Changes since v3:

* Do not make git sparse-checkout disable throw an error when not in a sparse-checkout -- and document why we don't exit early either.

Changes since v2:

* Small documentation wording improvement
* Added Reviewed-by's from Stolee and Victoria

Changes since v1:

* Inserted new patches 3 & 4 as additional preparatory cleanups
* Took the new mode-toggling work code in sparse_checkout_set from the previous series and moved it into a new function, as a preparatory patch, and made it usable by init/set/reapply
* Also updated reapply to allow mode-toggling
* Updated the documentation as per above
* Various other small items from review comments

A quick overview:

* Patches 1-4: small preparatory refactorings
* Patch 5: patch adding new function for toggling {cone,sparse-checkout} modes
* Patch 6: the crux of the series; make set able to handle both init and set options
* Patch 7: make reapply also able to do mode toggling
* Patches 8-9: documentation modifications (Patch 4 is worth reviewing; it marks init as deprecated -- are others okay with that?)
* Patch 10: trivial modification of git clone --sparse to use git sparse-checkout set rather than git sparse-checkout init.

[1]
https://lore.kernel.org/git/CABPp-BE8TJ8QGAQWsSGT7S+9Xp-XmApcC9PSw3K=RQOP0rt+PQ@mail.gmail.com/

Elijah Newren (10):
sparse-checkout: pass use_stdin as a parameter instead of as a global
sparse-checkout: break apart functions for sparse_checkout_(set|add)
sparse-checkout: add sanity-checks on initial sparsity state
sparse-checkout: disallow --no-stdin as an argument to set
sparse-checkout: split out code for tweaking settings config
sparse-checkout: enable set to initialize sparse-checkout mode
sparse-checkout: enable reapply to take --[no-]{cone,sparse-index} git-sparse-checkout.txt: update to document init/set/reapply changes
Documentation: clarify/correct a few sparsity related statements
clone: avoid using deprecated sparse-checkout init

 Documentation/git-clone.txt           |   8 +-
 Documentation/git-sparse-checkout.txt | 100 +++++++------
 builtin/clone.c                       |   2 +-
 builtin/sparse-checkout.c             | 204 ++++++++++++++++++++------
 t/t1091-sparse-checkout-builtin.sh    |  10 +-
 5 files changed, 227 insertions(+), 97 deletions(-)

base-commit: abe6bb3905392d5eb6b01fa6e54d7e784e0522aa
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1151%2Fnewren%2Fsparse-checkout-no-init-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1151/newren/sparse-checkout-no-init-v4
Pull-Request: https://github.com/git/git/pull/1151

Range-diff vs v3:

  1:  814aed2d125 =  1:  814aed2d125 sparse-checkout: pass use_stdin as a parameter instead of as a global
  2:  04cd57592e9 =  2:  04cd57592e9 sparse-checkout: break apart functions for sparse_checkout_(set|add)
  3:  f3af5edb25d !  3:  a8fdda35b91 sparse-checkout: add sanity-checks on initial sparsity state

@@ Metadata
## Commit message ##
sparse-checkout: add sanity-checks on initial sparsity state

- Most sparse-checkout subcommands (list, add, reapply, disable)
- only make sense when already in a sparse state. Add a quick check
- that will error out early if this is not the case.
+ Most sparse-checkout subcommands (list, add, reapply) only make sense
+ when already in a sparse state. Add a quick check that will error out
+ early if this is not the case.
+
+ Also document with a comment why we do not exit early in disable even
+ when core.sparseCheckout starts as false.

@@ builtin/sparse-checkout.c: static int sparse_checkout_disable(int argc, const ch
struct pattern_list pl;
struct strbuf match_all = STRBUF_INIT;

-+ if (!core_apply_sparse_checkout)
-+ die(_("no active sparse-checkout to disable"));
++ /*
++ * We do not exit early if !core_apply_sparse_checkout; due to the
++ * ability for users to manually muck things up between
++ * direct editing of .git/info/sparse-checkout
++ * running read-tree -m u HEAD or update-index --skip-worktree
++ * direct toggling of config options
++ * users might end up with an index with SKIP_WORKTREE bit set on
++ * some files and not know how to undo it. So, here we just
++ * forcibly return to a dense checkout regardless of initial state.
++ */
+
argc = parse_options(argc, argv, NULL,
builtin_sparse_checkout_disable_options,

  4:  82a13cc0197 =  4:  5882332b97f sparse-checkout: disallow --no-stdin as an argument to set
  5:  7a812e0222c =  5:  3e9e28c8dd2 sparse-checkout: split out code for tweaking settings config
  6:  7167a4b3118 =  6:  595ba138603 sparse-checkout: enable `set` to initialize sparse-checkout mode
  7:  3687637915f =  7:  09b13280c26 sparse-checkout: enable reapply to take --[no-]{cone,sparse-index}
  8:  7483d1363e5 =  8:  9d96da855ea git-sparse-checkout.txt: update to document init/set/reapply changes
  9:  11a45920602 =  9:  f669829a98b Documentation: clarify/correct a few sparsity related statements
 10:  395d9b194d3 = 10:  ae671aa615a clone: avoid using deprecated `sparse-checkout init`

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