Support for the histogram diff algorithm

[wetneb]

In Git, the default diff algorithm used when merging is not the Myers algorithm implemented here, but the so-called histogram algorithm, which can significantly improve the quality of merges.

To see an example where the diff algorithm makes a difference to the merged output, see for instance the test cases in this PR, which shows a case where using Myers creates various conflicts, while using histogram merges cleanly.

I wonder if implementing the histogram algorithm (in addition to Myers) would be in scope for this library?

[wetneb]

This has been implemented in a different crate: imara-mergy, which did so by vendoring parts of diffy and integrating it with imara-diff.

[wetneb]

A newer crate diffy-imara is available, with all of diffy's functionality, plus histogram diff support.