Fix git show rev:path output formatting and duplication

[stringer07]

Title: Fix duplicate and truncated output for git show rev:path commands

Description

Motivation and Context
Previously, when using this tool to view the contents of a specific file at a specific revision (e.g., git show <rev>:<path>), the tool failed to recognize it as a raw blob request. Instead, it treated the output as a standard commit diff.

This resulted in two critical bugs:

1. Duplicated Output: Because the raw file blob was incorrectly passed down to the diff formatting/compacting logic, the file contents were printed multiple times.
2. Corrupted File Content: In the fallback passthrough mode, the output was unconditionally printed using println!("{}", stdout.trim());. This stripped all intentional leading/trailing empty lines and forced an unwanted trailing newline.

Steps to Reproduce the Bug:
Running a command piped to grep, such as:

rtk git show develop:modules/pairs_backtest.py | grep -n "def _align_arrays"

Expected: A single line showing the matched function signature.
Actual: Duplicate matches returned because the wrapper output the entire file contents twice.

Changes Proposed

• Added is_blob_show_arg: A helper function to detect the rev:path syntax (arguments containing : and not starting with -).
• Introduced wants_blob_show flag: When a blob request is detected, the command execution routes to a strict passthrough mode.
• Fixed Output Formatting: For blob outputs, the code now uses print!("{}", stdout); instead of println!("{}", stdout.trim());, ensuring the raw file content is preserved exactly as Git outputs it.
• Added Unit Tests: Included test_is_blob_show_arg to verify the argument parsing logic handles branches, paths, and formatting flags correctly.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring

Testing

• Added unit tests for argument detection (test_is_blob_show_arg).
• Manually verified that rtk git show develop:modules/pairs_backtest.py | grep now correctly outputs exactly one match, and file trailing newlines are perfectly preserved.