Feature finish should not fail on already merged feature branch.

[rostskadat]

Description:

Call to feature-finish fails when the feature branch has been already merged. It fails with:

[ERROR] Failed to execute goal com.amashchenko.maven.plugin:gitflow-maven-plugin:1.14.1-SNAPSHOT:feature-finish (default-cli) on project TestProject: feature-finish: On branch develop
[ERROR] Your branch is up to date with 'origin/develop'.
[ERROR]
[ERROR] nothing to commit, working tree clean

Steps To Reproduce:

1- Create a feature branch on the TestProject
2- Do some development on that feature branch
3- Simulate an "delivery" (possibly intermediate) by merging the feature branch into develop
4- Call feature-finish

Expected Result:

I would expect the feature-finish goal to:
1- Consider that it is a valid state for the feature branch.
2- Carry on with the rest of the process.

[aleksandr-m]

It would be nice to have this controlled by some parameter, but then we need to check diff or dry-run it first.

Related #296