[SCM-970] Have separate APIs for distributed and centralized version control

**[Robert Scholte](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=rfscholte)** opened **[SCM-970](https://issues.apache.org/jira/browse/SCM-970?redirect=false)** and commented

The nature of these 2 types are very different. The original scm api was based on centralized repositories, and the distributed was implemented behind those methods, which makes it more complex than required.

Splitting this will make it easier to make full use of distributed features.

Keep in mind that plugins like the maven-release-plugin must still be able to do their work, but they may need to provide implementations for both tastes (unless the original scm-api implements it).

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**Issue Links:**
- [SCM-852](https://issues.apache.org/jira/browse/SCM-852) maven-scm-plugin:check-local-modification should optionally check for unpushed commits

- [SCM-996](https://issues.apache.org/jira/browse/SCM-996) Streamline checkIn operation in git providers with respect to modified/added/removed files

- [MRELEASE-1030](https://issues.apache.org/jira/browse/MRELEASE-1030) Introduce flag to push once, not twice, during release:prepare
(_**"is depended upon by"**_)
- [MRELEASE-954](https://issues.apache.org/jira/browse/MRELEASE-954) maven-release-plugin should push less often to git server
(_**"is depended upon by"**_)

1 votes, 3 watchers

No files, tests, or entry points are named. Start by reviewing the current SCM API and the linked SCM-852 and SCM-996 issues, then trace how maven-release-plugin depends on it; done means defined separate centralized and distributed APIs while preserving required release-plugin work.

Written by the indexing model from the issue text.