fix: add reftable HEAD migration hint

[puneetdixit200]

Related to #1743.

This adds a targeted hint for the reftable placeholder failure discussed in the issue. When the commit traversal error chain shows HEAD resolving through refs/heads/.invalid or Reference name cannot start with a dot, onefetch now keeps the original error chain and adds a concrete recovery step:

git refs migrate --ref-format=files

This does not try to implement reftable support in onefetch. It just turns the current low-level gix failure into an actionable message while that support is still pending upstream.

Tests cover both the reftable-style error chain and an unrelated error that should not receive the hint.

Checks run locally:

• rustfmt --check src/info/mod.rs
• CARGO_HOME=$PWD/.cargo-home CARGO_TARGET_DIR=$PWD/target cargo test info::tests::test_add_reftable_head_hint --lib
• CARGO_HOME=$PWD/.cargo-home CARGO_TARGET_DIR=$PWD/target cargo test test_display_head_refs --lib
• CARGO_HOME=$PWD/.cargo-home CARGO_TARGET_DIR=$PWD/target cargo test --test repo test_repo -- --exact
• CARGO_HOME=$PWD/.cargo-home CARGO_TARGET_DIR=$PWD/target cargo clippy --lib -- -D warnings

[spenserblack]

I think it would've been better to match on the gix error, so that cheap pattern and value matching could be done instead of string matching on error messages.

But overall this is good enough IMO.