Fix hard-to-reproduce race condition in SpotlessCheck.

[nedtwigg]

I've never seen this bug on Win or Linux, but it started happening on my mac dev box today. GitRatchetGradleTest can occasionally fail, in the following way. This line was failing, claiming that a file was dirty. But the file was actually formatted correctly, and just leftover in the build/spotless/<formatName> directory. It only fails 1 out of 3 or 4 invocations.

spotless/plugin-gradle/src/test/java/com/diffplug/gradle/spotless/GitRatchetGradleTest.java

Lines 203 to 204 in de2d84e

	// now it is "git dirty" but "format clean"
	assertPass("spotlessCheck");

There is an easy fix, which is for spotlessCheck to double-check that files in build/spotless/<formatName> really are different than what is on disk. It needs to do this anyway in order to generate the format violations, so it might as well be eager about it.

Given how difficult it is to trigger, triggering the bug is likely to

• require a low-timestamp-resolution-filesystem
• caching with both JGit and Gradle
• coupled with doing multiple independent gradle invocations all without crossing 1 second timestamp boundary

In short, an easy workaround is probably the right answer :)