repository: keep the store cache out of repo-create and repo-delete, refs #10396

[mr-raj12]

Description

Implements option A of #10396: repo-create and repo-delete only act on the repository, not on the BORG_STORE_CACHE directory. refs #10396.

With BORG_STORE_CACHE set (master a586f69, borgstore 0.7.0):

• borg repo-create failed with rc 11 ("has no repository config") as soon as the cache directory was not empty, and left an empty repository directory behind.
• borg repo-delete removed the whole cache directory: the cached packs of all other repositories and any other file in there.

Changes:

• Repository(create=True) does not give a cache to the Store.
• Repository.destroy() and the cleanup after a failed create destroy the store's backend only (Repository._destroy_store).
• Help text of BORG_STORE_CACHE: cached packs of a deleted repository stay until they are evicted or the directory is removed.

Tests:

• test_create_with_a_filled_store_cache (file, ssh): creating a repository leaves a filled cache directory unchanged, the cache is used when the repository is opened afterwards.
• test_destroy_keeps_the_store_cache (file, ssh): destroy() removes the repository, the cache directory keeps all its files.
• test_create_failure_keeps_the_store_cache: a failing save_config / open after the store was created removes the store, the cache directory keeps all its files.
• test_repo_create_with_a_filled_store_cache: a second borg repo-create works.
• test_delete_repo_keeps_the_store_cache: borg repo-delete of one repository keeps the cache files, the other repository still extracts.
• test_gather_many_one_gather_for_many_packs and test_salvage_pack_refuses_with_a_pack_store_cache now reopen the repository after creating it, so they still run with the cache.

The create and destroy tests fail on master. test_destroy_keeps_the_store_cache and test_delete_repo_keeps_the_store_cache also fail if only _destroy_store calls Store.destroy().

Not in this PR: a per-repository cache directory (option B) and a check for a cache directory that holds other content. Touches the same lines of Repository.__init__ as #10467.

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.74%. Comparing base (e84d7ac) to head (09613dc).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10477   +/-   ##
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  Coverage   88.73%   88.74%           
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  Files         103      103           
  Lines       19392    19394    +2     
  Branches     3024     3024           
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+ Hits        17208    17211    +3     
+ Misses       1515     1514    -1     
  Partials      669      669           

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