Revert "Use a safer command for rebase"

[Kobzol]

Reverts rust-lang/triagebot#1808. As found by RalfJ, we cannot hardcore this message for a specific repository :man_facepalming: A silly mistake on my part.

[ehuss]

@Kobzol If you want to make this happen, the URL for the repository can be obtained with something like event.issue.repository().url(&ctx.github).