Use a safer command for rebasing

[Kobzol]

In a similar vein to rust-lang/rustc-dev-guide#1889.

[rustbot]

r? @jackh726

rustbot has assigned @jackh726.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[ehuss]

Updating this sounds great. However, users are probably more likely to see the message posted by the bot (source in no_merges.rs), which would probably be good to keep in sync here.

[Kobzol]

Did that here.

[ehuss]

Thanks!