update_plugin: clashes with git config pull.rebase

[serl]

Hello,
In my global git configuration, I set pull.rebase to interactive (more info in git documentation for git pull and for git config)
While this might be a quite extravagant configuration, it breaks the updates for tpm, as it relies on the fact that git pull is a non-interactive command. Which is definitely not in my weird configuration.

Explicitly passing the --rebase=false should force the default behavior. It fixes the issue in my configuration and should have no impact on others.

Thanks! :)