x/build/cmd/gerritbot: perform gofmt checks before pushing to Gerrit

[tklauser]

In https://golang.org/cl/99337 a file using CRLF instead of LF line endings was added. When using git-codereview to send CLs, there is a gofmt check which prevents this. But it seems there is no corresponding check when sending CLs via Github PRs.

It would be nice to run the gofmt check also for PRs submitted via Github.

/cc @bradfitz @andybons

[gopherbot]

Change https://golang.org/cl/114255 mentions this issue: debug/pe: gofmt

[tklauser]

There was a similar issue with https://golang.org/cl/110555

[bradfitz]

I still would rather prioritize #18548

[gopherbot]

Change https://golang.org/cl/115356 mentions this issue: os/exec: gofmt