vfmt: exit with error code if encountering diffs with -diff flag

[ttytm]

Currently, it's hard to combine a diff-output while also verifying that paths are formatted.

Up to now I'm doing contortions like:

v fmt -verify <path> && exit 0 || v fmt -diff <path> && exit 1

v fmt -verify -diff is not possible either, since the single flags do return early.

With the changes, the -diff flag will cause an exit code 1 if diffs are encountered. This is equivalent to diff tools like diff.