Reorganize refs optimization tests to properly handle backend-specific vs backend-agnostic tests

Context

Currently, t/t1463-refs-optimize.sh enforces the files ref format for all tests using:

GIT_TEST_DEFAULT_REF_FORMAT=files
export GIT_TEST_DEFAULT_REF_FORMAT

This test file sources pack-refs-tests.sh, which is also used by t/t0601-reffiles-pack-refs.sh.

Problem

1. The refs optimize command isn't backend-specific, but t1463 currently enforces the files format
2. The naming doesn't reflect that the test is backend-specific
3. Adding reftable-specific tests to the current structure is awkward
4. The shared test file pack-refs-tests.sh has confusing naming given its usage

Proposed Solution

Two potential approaches discussed:

Option 1: Keep t1463 backend-agnostic

• Remove GIT_TEST_DEFAULT_REF_FORMAT=files from t1463
• Add REFFILES prerequisite to existing tests in pack-refs-tests.sh
• Add reftable-specific tests using git init --ref-format=reftable

Option 2: Move to t06* range and create separate test files

• Move t1463 to the t06* range (backend-specific tests)
• Introduce a separate reftable-specific test file for refs optimization
• Create a new backend-generic test suite if needed

Recommendation

Option 2 is preferred as it:

• Properly reflects that current tests are backend-specific through naming/location
• Keeps backend-specific tests in the t06* range where they belong
• Allows for cleaner separation between reffiles and reftable tests

Related Files

• t/t1463-refs-optimize.sh
• t/t0601-reffiles-pack-refs.sh
• t/pack-refs-tests.sh

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Discussion from Slack thread