Path filters: Recurse into directories if child entries may be included by filters

[qmfrederik]

GitExtensions.IsRelevantCommit will determine whether a commit is relevant when calculating version height. A commit is considered relevant if any of the files which are changed in that commit match the path filters.

This code relies on FilterPath.Includes to determine whether a specific path is included in by a path filter.

The unmanaged implementation gets a list of full file names, whereas the managed implementation recurses the directory tree.

This causes a bug where the managed implementation would not recurse into directories whose child items may be included.
For example, if you have a path filter to include src/lib, FilterPath.Includes("src") would return false, and changes to any files in src/lib/ would not be considered.

This should fix the root cause of #587.

[qmfrederik]

@AArnott I updated the unit test you created to use / as a path separator instead of \.

[AArnott]

Thanks. I rebased your commits to get them so they can target v3.4 instead of master.