[SCM-852] maven-scm-plugin:check-local-modification should optionally check for unpushed commits

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[SCM-852](https://issues.apache.org/jira/browse/SCM-852?redirect=false)** and commented

Currently the goal `check-local-modification` only checks for local changes. But with distributed SCM systems it is very often also desirable to enforce that all local commits have been pushed to the origin as well. It would be good to optionally check for unpushed commits or not yet fetched commits with that goal.

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**Affects:** 1.9.5

**Issue Links:**
- [SCM-970](https://issues.apache.org/jira/browse/SCM-970) Have separate APIs for distributed and centralized version control

Start by locating the maven-scm-plugin goal check-local-modification and read the linked SCM-970 discussion about separate distributed and centralized version-control APIs. Define how an optional check should cover unpushed or not-yet-fetched commits across supported distributed SCM systems, then verify the goal's existing behavior and add coverage for the selected option.

Written by the indexing model from the issue text.