Added DirectoryRepo as a GitRepo substitute, for non-git VCS support with repomap etc.

[QuadrupleA]

I use and prefer the fossil version control system to git; sadly, Aider unnecessarily disables basic features like building the RepoMap or listing/iterating files in the current or command-line directory when a git repo is not present, rather than just obviously unsupportable features like /commit, /undo, etc.

This PR creates a DirectoryRepo object, as an alternative for GitRepo when a git repo is not found, which allows Aider to iterate the files in the directory in --no-git mode (using either the startup CWD, or the single directory passed on command line). It handles & respects the .aiderignore file.

This restores the functionality of /map, /ls, etc. and allows it to work well with other version control systems (e.g. with manual commits / undos via /run fossil revert or /run fossil commit -m "...").

At launch, with --no-git in a non-git directory, Aider will now show:

Aider v0.75.1
Main model: anthropic/claude-3-7-sonnet-20250219 with diff edit format, prompt cache, infinite output
Weak model: anthropic/claude-3-5-haiku-20241022
Git repo: non-git directory with 14 files
Repo-map: using 4096 tokens, files refresh

[CLAassistant]

All committers have signed the CLA.

[academo]

This is a very necessary feature. A lot of the times I want to use aider I am experimenting with code and I don't want to go through the hassle of having to add new experimental files to git and restart aider just for it to decide to work.