Unified Interface for Differencers

[pmbittner]

Currently, accessing different differencers is a bit messy:

VariationTree<DiffLinesLabel> s1 = VariationTree.fromFile(pathBefore);
VariationTree<DiffLinesLabel> s2 = VariationTree.fromFile(pathAfter);

VariationDiff<DiffLinesLabel> diffViaGit, diffViaGumTree, diffViaTrueDiff;
diffViaGit = VariationDiff.fromFiles(pathBefore, pathAfter,
    DiffAlgorithm.SupportedAlgorithm.MYERS,
    VariationDiffParseOptions.Default);
diffViaGumTree  = VariationDiff.fromTrees(s1, s2);
diffViaTrueDiff = TrueDiffDetective.compare(s1, s2);

It would be more transparent if we would have unified interfaces for differencing algorithms such as this for example:

public interface TextBasedDifferencer<L extends Label> {
    public VariationDiff<L> diff(String before, String after);
}

public interface TreeBasedDifferencer<L extends Label> {
    public VariationDiff<L> diff(VariationTree<L> before, VariationTree<L> after);
}

Then methods could rely on these differencers instead of having lots of convencience methods all around the place as of now.