When the working tree/stage is different from the current HEAD commit -> generate SNAPSHOT version

## Enhancement

when I am developping and I did not yet commit my work, I would that this plugin generate a version snapshot, like x.y.(z+1)-SNAPSHOT

My use case :

- `d357f6c - (HEAD -> master, tag: v2.26.0, origin/master, origin/HEAD)`
- `mvn -q -Dexec.executable=echo -Dexec.args='${project.version}' --non-recursive exec:exec` -> 2.26.0
- I do some modifications on code without commit. `git status`
- `mvn -q -Dexec.executable=echo -Dexec.args='${project.version}' --non-recursive exec:exec` -> 2.26.0. Whereas I would have 2.26.1-SNAPSHOT as version generated because the stage/working tree has been different that the current HEAD commit.