Allow users to upgrade to arbitrary github remote

[robbintt]

This change allows users to upgrade to an arbitrary github remote.

• Update --install-main-branch to take an optional url
• rename it to --update-branch to be more generic.

Usage Example

(aider-dev) 🔥 aider --install-branch git+https://github.com/robbintt/aider.git@robbintt-arbitrary-branch-install
─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Install the development version of aider from 
git+https://github.com/robbintt/aider.git@robbintt-arbitrary-branch-install?

/home/robbintt/virtualenvs/aider-dev/bin/python3 -m pip install --upgrade --upgrade-strategy only-if-needed 
git+https://github.com/robbintt/aider.git@robbintt-arbitrary-branch-install
Run pip install? (Y)es/(N)o [Yes]:                                                                                       

Installing: /home/robbintt/virtualenvs/aider-dev/bin/python3 -m pip install --upgrade --upgrade-strategy only-if-needed git+https://github.com/robbintt/aider.git@robbintt-arbitrary-branch-install
                
Installation complete.

[CLAassistant]

All committers have signed the CLA.

[robbintt]

Planning to add an arbitrary "downgrade" option, although it may be possible by directly "upgrading" to a versioned git tag using this PR.

[robbintt]

uv may allow the user to downgrade back to releases, haven't tested it.