git-add: Skip submodules with ignore=all unless --force and explicit path used

The ignore=all configuration for submodules is currently honored by status and diff, but not by add. This inconsistency causes friction when working with submodules:

* Developers unintentionally stage submodule updates, leading to conflicts and unnecessary changes.
* Tracking branches in submodule configs causes friction as updates to the submodule SHA1 are added without intent.

This patch aligns git add with the behavior of status and diff:

* Submodules marked ignore=all are skipped by default.
* Attempting to add such a submodule explicitly now requires --force.

A submodule can frictionless track a branch in the configuration or manually switched to and use update --update --remote without unintentionally add the changes to the super-project.

Tests that previously added submodules without --force have been updated accordingly. A new test is added to test the behavior of git add with and without --force.

Claus Schneider(Eficode) (5):
read-cache: update add_files_to_cache take param ignored_too
read-cache: submodule add need --force given ignore=all configuration
tests: t2206-add-submodule-ignored: ignore=all and add --force tests
tests: fix existing tests when add an ignore=all submodule
Documentation: update add --force option + ignore=all config

 Documentation/config/submodule.adoc |  13 +--
 Documentation/git-add.adoc          |   5 +-
 Documentation/gitmodules.adoc       |   5 +-
 builtin/add.c                       |   2 +-
 builtin/checkout.c                  |   2 +-
 builtin/commit.c                    |   2 +-
 read-cache-ll.h                     |   2 +-
 read-cache.c                        |  80 ++++++++++++++++-
 t/lib-submodule-update.sh           |   6 +-
 t/meson.build                       |   1 +
 t/t2206-add-submodule-ignored.sh    | 134 ++++++++++++++++++++++++++++
 t/t7508-status.sh                   |   2 +-
 12 files changed, 235 insertions(+), 19 deletions(-)
 create mode 100755 t/t2206-add-submodule-ignored.sh

base-commit: 9a2fb147f2c61d0cab52c883e7e26f5b7948e3ed
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1987%2FPraqma%2Frespect-submodule-ignore-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1987/Praqma/respect-submodule-ignore-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/1987

Range-diff vs v3:

 1:  d2b66286c2 = 1:  69e0065b1b read-cache: update add_files_to_cache take param ignored_too
 2:  de72bf0a91 ! 2:  504edcb0f5 read-cache: submodule add need --force given ignore=all configuration

@@ Commit message
unless overridden by --force and the submodule path is explicitly specified.

A message is printed (like ignored files) guiding the user to use the
- --force flag if the user has explicitely want to update the submodule
- reference.
+ --force flag if the user explicitly wants to update the submodule reference.

- The reason for the change is support submodule branch tracking or
- similar and git status states nothing and git add should not add either
- as a default behaviour. The workflow is more logic and similar to regular
- ignored files even the submodule is already tracked.
+ The reason for the change is to support branch tracking in submodules
+ with configuration submdule.<name>.branch or similar workflows where the
+ user is not interested in tracking each update of the sha1 in the submdule.
+ You can additionally set submodule.<name>.ignore=all and the git status
+ will state nothing and, with this patch, the git add does not either - as
+ the default behaviour. This patch changes the workflow to a more logical
+ behaviour and similar to workflow for ignored files.

- The change opens up a lot of possibilities for submodules to be used
- more freely and simular to the repo tool. A submodule can be added for many
- more reason and loosely coupled dependencies to the super repo which often
- gives the friction of handle the explicit commits and updates without
- the need for tracking the submodule sha1 by sha1.
+ The patch gives more scenarios for submodules to be used effectively with
+ less friction similar to the "repo" tool. A submodule can be added for many
+ different reasons than a hard dependency. It can be added as loosely
+ coupled dependencies whereas the user wants the latest based on the
+ configuration submoule.<name>.branch, but are not interested to track
+ each commit in the super-repo. Currently it gives friction of handling
+ conflicts between branches even the sha1's are fast-forward and the user
+ just wants the latest in any way. The user can still add a sha1 explicitly
+ to track updates.

 3:  d7b97e12d3 = 3:  d5ea8eb6e0 tests: t2206-add-submodule-ignored: ignore=all and add --force tests
 4:  6e44ab6804 = 4:  df5c85d9f2 tests: fix existing tests when add an ignore=all submodule
 5:  37fe7956e0 ! 5:  8bfaa14d44 Documentation: update add --force option + ignore=all config

@@ Documentation/config/submodule.adoc: submodule.<name>.fetchRecurseSubmodules::
- commit when it has been staged), "dirty" will ignore all changes
- to the submodule's work tree and
+ a submodule as modified.
-+ Set to "all" will never consider the submodule modified. It can
++ When set to "all" will never consider the submodule modified. It can
+ nevertheless be staged using the option --force and it will then show up
+ in the output of status.
-+ Set to "dirty" will ignore all changes to the submodule's work tree and
++ When set to "dirty" will ignore all changes to the submodule's work tree and
takes only differences between the HEAD of the submodule and the commit recorded in the superproject into account. "untracked" will additionally let submodules with modified tracked files in their work tree show up.
- Using "none" (the default when this option is not set) also shows
- submodules that have untracked files in their work tree as changed.
-+ Set to "none"(default) It is also shows submodules that have untracked
-+ files in their work tree as changed.
++ When set to "none"(default) It also show submodules as changed if they have
++ untracked files in their work tree.
This setting overrides any setting made in .gitmodules for this submodule, both settings can be overridden on the command line by using the "--ignore-submodules" option. The 'git submodule' commands are not

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