fix(completion): handle -o {default,dirnames,plusdirs}

[reubeno]

• Fills out handling of default, dirnames, and plusdirs options for completion.
• Adds a few basic compat tests for this logic.

Notably, this makes git diff -- <start_of_file_path> complete to something sensibly.

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

brush-core/src/completion.rs:152

• The option 'dir_names' is inconsistently named compared to other options like 'bash_default' and 'default'. It should be renamed to 'dirnames' for consistency.

pub dir_names: bool,

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.51 μs	3.51 μs	0.00 μs	⚪ Unchanged
for_loop	933336.71 μs	942287.18 μs	8950.47 μs	🟠 +0.96%
instantiate_shell	61.57 μs	62.04 μs	0.47 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	28398.82 μs	27951.87 μs	-446.96 μs	⚪ Unchanged
parse_bash_completion	2814.61 μs	2874.32 μs	59.71 μs	🟠 +2.12%
parse_sample_script	4.21 μs	4.38 μs	0.17 μs	🟠 +4.02%
run_echo_builtin_command	90.99 μs	91.80 μs	0.81 μs	⚪ Unchanged
run_one_builtin_command	111.62 μs	111.38 μs	-0.24 μs	⚪ Unchanged
run_one_external_command	1751.25 μs	1719.36 μs	-31.89 μs	🟢 -1.82%
run_one_external_command_directly	824.27 μs	824.08 μs	-0.19 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/complete.rs	🟠 56.43%	🟠 60.71%	🟢 4.28%
brush-core/src/completion.rs	🔴 28.42%	🔴 29.67%	🟢 1.25%
brush-core/src/shell.rs	🟢 77.73%	🟢 77.86%	🟢 0.13%
Overall Coverage	🟢 74.81%	🟢 74.97%	🟢 0.16%

Minimum allowed coverage is 70%, this run produced 74.97%

[github-actions]

Test Results

    2 files      9 suites   1m 16s ⏱️
  569 tests   569 ✅ 0 💤 0 ❌
1 124 runs  1 124 ✅ 0 💤 0 ❌

Results for commit 5812a47.