rc tools git: command for easy recursive blaming

Our ":git blame" annotates each line with the most recent commit.
However often a line has been modified by several commits.

Introduce ":git blame-jump" which shows the commit that added the
line at cursor. Crucially, it works also in Git diff buffers, so it
can be used recursively to find the full history of a line.

To do the recursive blame from a diff, I need to navigate to the
old (deleted) version of a line. Since old and new line are usually
neighbors. Speed up the common scenario of finding the old version
by making ":git blame-jump" jump to the new version. This means the
initial diff view might not include the commit message etc. Compensate
this by showing the commit's date+author+subject in the status line.

Here are some test cases.
- run blame-jump after "git blame"
- create an uncommitted or unsaved line, run "git blame" and
  "blame-jump" on the uncommitted line
- run blame-jump without running "git blame"
- run blame-jump in "git show"
- run blame-jump in "git diff HEAD"
- run blame-jump in "git diff --cached"
- run blame-jump in "git diff" (YMMV if there are cached changes,
  could fix that)

Naming: there are some similar commands in the wild [1];
they are usually called "show-blamed" or similar, but they
don't jump to the corresponding line.  Also our list of git
commands is getting a bit messy (especially the undocumented
show-diff/hide-diff/next-hunk/prev-hunk; subject first naming seems
better).

[1]: https://github.com/robertmeta/kak/blob/f6e78ec4c0eeccd091e6275828234d98e6aa3a7f/kakrc#L423

Future work: to go back to the previously-blamed commit we need to
have had the foresight to use "rename-buffer". Perhaps we want to
add some kind of buffer stack (like Magit does for example).

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