git-like git-annex diff

It took me some trial and error to get git-annex diffdriver to work close to the way I wanted (basically, the same way git diff works in a normal git repository with color.ui set to always with git config, so I thought I'd put my notes here for others:

Install colordiff (I couldn't figure out what git's actual default diff program was or if it was possible to use it outside of git diff, so I went with this.
Create a wrapper script to pass to git-annex diffdriver. LANG=C was to avoid a warning produced by perl when it runs colordiff. exit 0 is needed because colordiff/diff has a non-zero exit status when the files it is passed are not identical and git expects the external diff program to exit with status 0:
```
  #!/usr/bin/env bash                                                             
  LANG=C colordiff -c "$1" "$2"                                                   
  exit 0
```
Put the wrapper script somewhere on PATH, make it executable, and name it something like git-annex-diff-wrapper.
Configure the repo to use the wrapper: git config diff.external "git-annex diffdriver -- git-annex-diff-wrapper --"