Merge branch 'et/pretty-format-c-auto'

Merge branch 'et/pretty-format-c-auto'

The commands in `git log` family take %C(auto) in a custom format
string.  This unconditionally turned the color on, ignoring
--no-color or with --color=auto when the output is not connected to
a tty; this was corrected to make the format truly behave as
"auto".

* et/pretty-format-c-auto:
  format_commit_message: honor `color=auto` for `%C(auto)`