git-delta: update to 0.12.0.

There's quite a lot in this release, with contributions from 8 different
people.

Firstly a breaking change: some deprecated options are no longer supported:
in particular plus-color and minus-color. See #914. A description of this
change and what you need to do if you're affected by it is below.

Secondly, improvements, including:

- A (much-needed) short help text is now available: use delta -h. Delta is
  the proud possessor of 100 command-line options, so I think this is going
  to be helpful.
- git blame output now displays line numbers (thanks to @th1000s)
- Improvements to file and mode labels by @WayneD
- delta now has a user manual: https://dandavison.github.io/delta/. You can
  even create a PDF copy (print button in upper-right hand corner), and the
  README is correspondingly now a much easier read with the minimal required
  setup instructions and an overview of functionality with screenshots.

Finally some bug fixes, including

- File path hyperlinks (e.g. in diff and grep output) were previously
  sometimes incorrect, especially when delta was invoked from a directory
  other than the repo root.
- Some fixes to grep output parsing edge cases

Deprecated options no longer supported:

The options affected here have been deprecated for a couple of years; I hope
this doesn't cause you any trouble. Here are the details:

The most common case is that you have something like this in your ~/.gitconfig:

[delta]
    plus-color = my_plus_color minus-color = my_minus_color

Please change that to

[delta]
    plus-style = syntax my_plus_color minus-style = normal my_minus_color

Here's the list of deprecated options with a summary of how to update them:

minus-color            Deprecated: use `minus-style = normal my_background_color`
plus-color              Deprecated: Use `plus-style = syntax my_background_color`
minus-emph-color        Deprecated: use `minus-emph-style = normal my_background_color`
plus-emph-color        Deprecated: Use `plus-emph-style = syntax my_background_color`
highlight-removed      Deprecated: use `minus-style = syntax my_background_color`
commit-color            Deprecated: use `commit-style = my_foreground_color
                                        commit-decoration-style = my_foreground_color`
file-color              Deprecated: use `file-style = my_foreground_color
                                        file-decoration-style = my_foreground_color`
hunk-style              Deprecated: synonym of `hunk-header-decoration-style`
hunk-color              Deprecated: use `hunk-header-style = my_foreground_color
                                        hunk-header-decoration-style = my_foreground_color`
theme                  Deprecated: use `syntax-theme`

(fcambus)