Incremental check may incorrectly run against all source files instead of the correct source set

[davidnelson-hms]

Versions:
ktlint-gradle: 9.1.1
Gradle version: 6.1.1
Kotlin version: 1.3.61

The Check task filters out incremental changes of type DIRECTORY or REMOVED. If the only changes to a source set as reported by Gradle have those types, then the resulting set of source files after the filter is empty. This causes ktlint to be invoked with an args file that does not list any source files to check. In that case, the default behavior of ktlint is to check ALL source files in the current directory.

I believe the correct behavior in the case where Gradle reports incremental changes, but all such changes are filtered out, would be to set the didWork property to false and return without executing ktlint.

I will try to create a repro and possibly submit a PR when I have time.