Set the rootDir as / instead of // if the project is in the root of the filesystem

[rudolfv]

This PR is to fix a bug if a gradle project is in the root of a filesystem. Normally this would not be an issue as projects would are generally not at the root, but in the case of a Docker or other container, the situation could arise:

	Caused by: java.lang.IllegalArgumentException: Expected '/src/main/java/com/acme/MyApplication.java' to start with '//'
		at com.diffplug.spotless.FileSignature.subpath(FileSignature.java:203)
		at com.diffplug.spotless.extra.GitAttributesLineEndings$CachedEndings.endingFor(GitAttributesLineEndings.java:126)
		at com.diffplug.spotless.extra.GitAttributesLineEndings$RelocatablePolicy.getEndingFor(GitAttributesLineEndings.java:95)
		at com.diffplug.spotless.Formatter.computeLineEndings(Formatter.java:211)
		at com.diffplug.spotless.PaddedCell.calculateDirtyState(PaddedCell.java:203)
		at com.diffplug.spotless.PaddedCell.calculateDirtyState(PaddedCell.java:188)
		at com.diffplug.gradle.spotless.SpotlessTaskImpl.processInputFile(SpotlessTaskImpl.java:71)
		at com.diffplug.gradle.spotless.SpotlessTaskImpl.performAction(SpotlessTaskImpl.java:57)
		at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

I've read the instructions about adding to the CHANGES.md file, but I don't see a -SNAPSHOT section. I did see the below however, so I presume it is no longer necessary?

• We no longer publish -SNAPSHOT for every build to main, since we have good JitPack integration. (#508)

[nedtwigg]

Thanks for the fix!

If you update each of the changelogs (root, gradle, and maven) along these lines:

## [Unreleased]
### Fixed
* Fixed a bug which occurred if the root directory of the project was also the filesystem root ([#732](https://github.com/diffplug/spotless/pull/732))

then I'll push the release button.

[rudolfv]

Thanks for the update, @nedtwigg! I have made the requested changelog changes.

[nedtwigg]

Published in plugin-gradle 5.8.2 and plugin-maven 2.6.1.