contrib: show pull # in prompt for github-merge script

Puts the pull id in the prompt for the shell launched by `github-merge.sh`. This is useful as a reminder that you're *a)* in a special shell *b)* which pull you're working on.
```
user@asdf:/.../projects/bitcoin/bitcoin$ contrib/devtools/github-merge.sh 5427
Removing qa/rpc-tests/walletbackup.sh
Dropping you on a shell so you can try building/testing the merged source.
Run 'git diff HEAD~' to show the changes being merged.
Type 'exit' when done.

(5427)user@asdf:/.../projects/bitcoin/bitcoin$ 
```
The current solution is only triggered on Debian/Ubuntu, and makes use of the default PS1 building in `.bashrc` there. There may be a more general way, although I've tried overriding `RC1` manually then `bash --norc -i`, but that completely replaces the prompt instead of adding to it.