Add *.rs.bk to .gitignore file by default.

[pyfisch]

Rustfmt creates a backup of changed files with the file extension .rs.bk. Since the old files are already present in the vcs they should not be commited.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[alexcrichton]

Thanks for the PR! We've actually had this request before as well but it ended up being closed. Thoughts on that?

[alexcrichton]

Closing due to previous comment

[pyfisch]

Another issue to do this was just opened: #2949

You said at the previous PR:

I suspect that rustfmt is the tool here which may want to not generate *.bk files by default.

I agree that this will be better. I personally do not need the .rs.bk files since the changes are in git anyway. Should I open an issue for rustfmt? If rustfmt does not change this I would still like to have this .gitignore directive since it prevents users from accidentally committing the backups and it is really unlikely that there may be relevant files ending with .rs.bk.

[alexcrichton]

Oops forgot to close! But yeah I've opened an issue on rustfmt: rust-lang/rustfmt#1114