[SCM-998] Streamline file separator used in ScmFile.getPath()

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[SCM-998](https://issues.apache.org/jira/browse/SCM-998?redirect=false)** and commented

Currently the different `ScmManager` methods sometimes return the OS separator and sometimes only the forward slash in the `ScmFile` (used in the complex return object).

As forward slash is currently returned on all OSes for `ScmManager.checkIn(...)` all other methods like `ScmManager.add(...)` and `ScmManager.remove(...)` should behave the same. Also the javadoc at `ScmFile.getPath()` should be clarified in this regard.

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**Issue Links:**
- [SCM-807](https://issues.apache.org/jira/browse/SCM-807) JGit impl check-in fails unless the Maven project is in the working copy root
(_**"Discovered while testing"**_)

Start by reading ScmFile.getPath() and the ScmManager.checkIn(), add(), and remove() entry points mentioned in the issue. Compare the separators returned by these methods across operating systems, then clarify the getPath() Javadoc and verify that add() and remove() use the same forward-slash behavior as checkIn().

Written by the indexing model from the issue text.