Using coverage/coverageOff preserves settings overrides

[mtrampont]

Hi,

This PR addresses the issue #146 .
The problem was that the method Extracted.append actually drops the settings that were added with the set command.
You can see it here: https://github.com/sbt/sbt/blob/1.0.x/main/src/main/scala/sbt/Extracted.scala#L105
session.original is used instead of session.mergeSettings .

Instead of using Extracted.append I used SessionSettings.reapply.

I added a scripted test to check that coverage and coverageOff do not reset settings set with set.

When running the scripted tests, aggregate was failing because a call to coverageReport was missing (according to https://github.com/scoverage/sbt-scoverage#multi-project-reports).
Also, coverage-off is failing because of 0f9496d , but it should be fixed by PR #166 or PR #168.

[gslowikowski]

@mtrampont can you rebase and update this PR?

toggleCoverage method, after merge, should now look like:

  private def toggleCoverage(status: Boolean): State => State = { state =>
    val extracted = Project.extract(state)
    val currentProjRef = extracted.currentRef
    val newSettings = extracted.structure.allProjectRefs.flatMap(proj =>
      Seq(coverageEnabled in proj := status)
    )
    val appendSettings = Load.transformSettings(Load.projectScope(currentProjRef), currentProjRef.build, extracted.rootProject, newSettings)
    val newSessionSettings = extracted.session.appendRaw(appendSettings)
    SessionSettings.reapply(newSessionSettings, state)
  }

[gslowikowski]

Closed by #186