Don't save *.rs.bk if VCS is detected

[alexcrichton]

We've gotten a few issues on Cargo to add *.rs.bk to the default gitignore, but I personally like to keep that pretty clean and I'd prefer if rustfmt just didn't generate these files by default. Especially if git is detected, and perhaps if the working directory is clean, could bk files not be generated by default?

[tikue]

hear! hear!

[konstin]

Any progress on that? This should be as simple as checking whether git status --porcelain prints anything to stdout.

[nrc]

This is not really on my radar, priority-wise. If anyone would like to have a go at this I'd be happy to mentor.

[nrc]

We're on our way to emitting backup files by default, so I don't think this will be so much of an issue

[tshepang]

@nrc is this rejected?

[nrc]

@tshepang We don't create bk files by default any more, so I don't think it is that useful (if the user turns on the option, I assume they really want it, not VCS-dependent).

[tshepang]

oh, that's fine