install.sh: merge .gitconfig instead of overwriting

[janmoesen]

Intelligently merge .gitconfig with the existing file (if any) by adding to the existing config by looping the values from git config --file $TILDE_FILE.

In case of a conflict, ask what to do. Always ignore Tilde's user.name and user.email. (Not that they are in Tilde, anyway, because I did not want anyone accidentally committing as me. But I could re-add them, and not have to worry about that anymore because the other configuration values are copied, not the whole file.)