Message appears only if merge was unsuccessful and makes deleting
as quick as copypasting.
Check makes sure that supplied argument is actually an number, which
helps avoid waiting for git to eror out on "no remote branch".
Additionally, renamed `original` upstream to `upstream`, according to
@sudden6's suggestion.
This allows to close related to PR issues when PR submitter forgets to
include that in commit message, or add some other additional info.
E.g. `./merge-pr.sh 3033 'Fu fu fu. :3'` would result in:
```
Merge pull request #3033
Fu fu fu. :3
sudden6 (1):
update friend activity on groupchat events fixes#2848
```
Merge messages are composed of PR number + appended shortlog.
Script is supposed to work only if there are no merge conflicts.
Example usage: `./merge-pr.sh 1234`
