color.diff.<slot>::
Use customized color for diff colorization. `<slot>` specifies
which part of the patch to use the specified color, and is one
- of `plain` (context text), `meta` (metainformation), `frag`
+ of `context` (context text - `plain` is a historical synonym),
+ `meta` (metainformation), `frag`
(hunk header), 'func' (function in hunk header), `old` (removed lines),
`new` (added lines), `commit` (commit headers), or `whitespace`
(highlighting whitespace errors).