- '%Cgreen': switch color to green
- '%Cblue': switch color to blue
- '%Creset': reset color
-- '%C(...)': color specification, as described in color.branch.* config option
+- '%C(...)': color specification, as described in color.branch.* config option;
+ adding `auto,` at the beginning will emit color only when colors are
+ enabled for log output (by `color.diff`, `color.ui`, or `--color`, and
+ respecting the `auto` settings of the former if we are going to a
+ terminal)
- '%m': left, right or boundary mark
- '%n': newline
- '%%': a raw '%'