Support for recursive blame

[knuesel]

For example in tig (and similarly in magit) you can blame recursively like this: Start by blaming some file, go to some line. Then press , to show the blame view of the same region of the same file but at the parent commit. This can be repeated to travel further back in time, always showing the same region of file. Press < to go the other way (forward in time).

This is very handy when trying to understand the history of a particular piece of code. It would be great to have it supported in gitui!

[ilyagr]

A more discoverable shortcut for this might be pressing Shift-B again (since we're running Blame again on a different commit). Perhaps left arrow can go back (though perhaps we can think of something better than that).

As an aside, Tig works well for this, but I found its shortcuts hard to discover or remember. I did write them up in jonas/tig#1315 (comment).