[SCM-870] ScmProvider.checkOut() should honor fileSet.getFileList()

**[Ilya Basin](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=basinilya)** opened **[SCM-870](https://issues.apache.org/jira/browse/SCM-870?redirect=false)** and commented

If fileSet.getFileList() is not empty, the checkout commands should checkout only files requested.

For SVN and CVS it means that the common prefix of the fileList should be appended to the repo URL.

The git implementation should clone with --no-checkout and then checkout the files from the list.

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No further details from [SCM-870](https://issues.apache.org/jira/browse/SCM-870?redirect=false)

Start at ScmProvider.checkOut() and trace the SVN, CVS, and Git checkout implementations, focusing on fileSet.getFileList(). Compare how each provider constructs its checkout operation and verify behavior for a non-empty file list. Done means each mentioned provider checks out only the requested files while preserving the existing behavior for an empty list.

Written by the indexing model from the issue text.