support for gitconfig include directives

[felixhummel]

Issue

I have my git user.name set by using git config includes (docs):

$ git config --get user.name
Felix Hummel

Steps:

git init .
aider

Expected: Aider does nothing concerning user.name or user.email.

Actual: Aider outputs the following warning and writes .git/config

Update git name with: git config user.name "Your Name"
Update git email with: git config user.email "you@example.com"

It would be nice if aider would use git's parsing of its configuration git user.name and git user.email.

Version and model info

aider 0.69.1