[sparse] squash and rebase #6169 sparse checkout support

[VinnyOG]

Squashed and rebased this PR libgit2#6169.

Will report investigation as I test sparse functionality locally.

• get libgit2 test running
• verify that all test pass
• verify existing g8 tests pass
• write new g8 tests for sparse repos / update some existing ones to add this curveball in
• make list of features that core git supports with sparse checkout
• verify which of these are supported and which are not with this PR
• make a list of work that needs to be done in order for this to be 8th Wall ready

Observations

(g8 refers to a non-sparse support build and ng8 refers to a build with this PR included)

Becoming sparse

Starting in a clone of the code8 repo, i executed this command on master git sparse-checkout init --cone.

g8 takes a few seconds and shows all files as deleted and ng8 returns quickly with no changes.
Here, we notice that top-level files are included by default but there are no directories.

➜  niantic-code8 git:(sparsetesting) l
total 440
drwxr-xr-x  32 pawelczarnecki  staff   1.0K Aug 10 09:35 .
drwxr-xr-x  14 pawelczarnecki  staff   448B Jul 21 14:23 ..
-rw-r--r--   1 pawelczarnecki  staff   3.4K Aug 10 09:28 .bazelrc
-rw-r--r--   1 pawelczarnecki  staff     6B Apr 25 11:39 .bazelversion
-rw-r--r--   1 pawelczarnecki  staff   2.0K Apr  1 09:49 .clang-format
-rw-r--r--   1 pawelczarnecki  staff   4.9K Apr 25 11:39 .eslintrc.js
drwxr-xr-x  15 pawelczarnecki  staff   480B Aug 10 09:35 .git
-rw-r--r--   1 pawelczarnecki  staff    87K Apr 25 11:39 .gitattributes
-rw-r--r--   1 pawelczarnecki  staff   1.2K Aug 10 09:28 .gitignore
-rw-r--r--   1 pawelczarnecki  staff    28B Apr  1 09:49 .pylintrc
-rw-r--r--   1 pawelczarnecki  staff     0B Apr 25 11:39 BUILD
-rwxr-xr-x   1 pawelczarnecki  staff   2.2K Apr  1 09:50 PRBuilder.py
-rwxr-xr-x   1 pawelczarnecki  staff   220B Apr  1 09:50 PRESUBMIT
-rw-r--r--   1 pawelczarnecki  staff   2.7K Aug 10 09:28 README.md
-rw-r--r--   1 pawelczarnecki  staff    26K Aug 10 09:28 WORKSPACE
-rwxr-xr-x   1 pawelczarnecki  staff   1.3K Apr  1 09:50 awe-build-install.sh
-rw-r--r--   1 pawelczarnecki  staff   1.5K Apr 25 11:39 c8.natvis
-rwxr-xr-x   1 pawelczarnecki  staff   1.4K Aug 10 09:28 cache-builder.sh
-rw-r--r--   1 pawelczarnecki  staff   2.2K Aug 10 09:28 code8-gcp-readers.json
-rw-r--r--   1 pawelczarnecki  staff    71B Apr  1 09:50 code8_config.py
-rw-r--r--   1 pawelczarnecki  staff   128B Apr 25 11:39 eslint-local-rules.js
-rwxr-xr-x   1 pawelczarnecki  staff   742B Aug 10 09:28 hello-build-builder.sh
-rwxr-xr-x   1 pawelczarnecki  staff   282B Apr  1 09:50 linecount.sh
-rwxr-xr-x   1 pawelczarnecki  staff   1.7K Aug 10 09:28 nightly-builder.sh
-rw-r--r--   1 pawelczarnecki  staff   827B Apr 25 11:39 package.json
-rw-r--r--   1 pawelczarnecki  staff   2.5K Aug 10 09:28 platform_mappings
-rwxr-xr-x   1 pawelczarnecki  staff   382B Apr  1 09:50 presubmit-lint.sh
-rwxr-xr-x   1 pawelczarnecki  staff   513B Apr  1 09:50 presubmit.sh
-rwxr-xr-x   1 pawelczarnecki  staff   2.0K Aug 10 09:28 reality-builder.sh
-rw-r--r--   1 pawelczarnecki  staff    62B Apr  1 09:50 setup.cfg
-rw-r--r--   1 pawelczarnecki  staff     0B Aug 10 09:28 test_file_delete
-rw-r--r--   1 pawelczarnecki  staff   667B Apr 25 11:39 tsconfig.json

Adding sparse entries.

➜  niantic-code8 git:(sparsetesting) git sparse-checkout add reality/cloud/xrhome
➜  niantic-code8 git:(sparsetesting) git status
On branch sparsetesting
Your branch is up to date with 'origin/pawelczarnecki-sparsetesting'.

You are in a sparse checkout with 13% of tracked files present.

nothing to commit, working tree clean
➜  niantic-code8 git:(sparsetesting) ng8 status
➜  niantic-code8 git:(sparsetesting) l
total 440
( ... removed regular files)
drwxr-xr-x   4 pawelczarnecki  staff   128B Aug 10 09:41 reality

Syncing

When I synced, every file in the repo was checked out again.
Added GIT_CHECKOUT_REMOVE_SPARSE_FILES to the checkout_options.strategy everywhere we do a checkout.