fix(options): keep COLUMNS from enabling icons auto

[xfocus3]

Summary

• Use the actual stdout terminal state for automatic output behavior instead of deriving it from the configured terminal width.
• Keep COLUMNS and --width as width inputs, so wide piped output can still use grid layout while --icons=auto suppresses icons when stdout is not a terminal.
• Add regression coverage for COLUMNS with --icons=auto.
• Update git2 to 0.21 and adjust branch-name handling for the current API, clearing the security advisories reported for 0.20.x.

Fixes #1842.

Tests

• cargo fmt --check
• cargo test deduce_view_does_not_treat_columns_as_tty --lib
• cargo test --lib
• cargo test
• env COLUMNS=200 cargo run -- --icons=auto <one-file-directory> | od -An -tx1 -c emitted file\n