Running git blame signals intent to view the blamed commit. Let's make
`<ret>` to go to that commit+line as long as blame info is shown.
In diff buffers we already use `<ret>` for "diff-jump".
Like blame annotations, the blame-jump mapping lives in window scope.
This means it will not collide with "diff-jump" which is mapped in
buffer scope.
Add the mapping synchronously (unlike the rest of the git-blame code)
to perhaps allow the user to override the mapping:
git blame; map window normal <ret> ...
24bf123503