update-core: If we are updating core packages, set PS1 and

[DavidEGrayson]

I did some experiments on fresh installations of MSYS2 in order to figure out the best way to solve this problem:

https://github.com/Alexpux/MSYS2-packages/issues/373

It looks like we need to do two things: change PS1 (and probably PROMPT_COMMAND) in the update-core script itself, and also change update-core so that it gets run inside the user's shell. This patch does the first part.

The second part is easy and I can make a pull request for it later. Basically we would just add these lines to /etc/profile:

update-core() {
  source /usr/bin/update-core
}

[mingwandroid]

Good work!

[DavidEGrayson]

Thanks @mingwandroid!