refactor: enhance git command execution robustness in add_new_entry.py

[RinZ27]

Updating the GitManager class to avoid relying on shell expansion and shell=True when retrieving Solidity files. Using shell=True with string interpolation is generally a brittle practice, especially when dealing with path specifications. By switching to argument lists (array-based) and implementing manual .sol filtering in Python, we make the internal utility script significantly more stable and predictable across different environments.

Running git ls-files and git diff directly with the target directory ensures that the file discovery logic remains precise. I've also added basic error handling for subprocess calls to prevent the script from crashing if a git command fails. These changes keep the maintainer workflow robust while maintaining the original functionality of the script.