Blame Extremely Slow

[shanedasilva]

I'm making heavy use of Blame to analyze every file patch in a commit. The larger the repository, the slower blame is.

I'm happy to contribute but looking for some more familiar with the library to give some advice before getting started.

[smola]

Our current blame implementation is a bit outdated (2015) and it was implemented before we had some important features related to diff'ing. This mean it does not take advantage of many later performance optimizations.

I think speeding it would require rewriting it from scratch leveraging DiffTree and Patch.

This #806 will need to be fixed too, to use the same diff implementation as other code paths.