Re: [PATCH v4 14/21] diff: add an internal option to dual-color diffs of diffs

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>
> When diffing diffs, it can be quite daunting to figure out what the heck
> is going on, as there are nested +/- signs.
>
> Let's make this easier by adding a flag in diff_options that allows
> color-coding the outer diff sign with inverted colors, so that the
> preimage and postimage is colored like the diff it is.
>
> Of course, this really only makes sense when the preimage and postimage
> *are* diffs. So let's not expose this flag via a command-line option for
> now.
>
> This is a feature that was invented by git-tbdiff, and it will be used
> by `git range-diff` in the next commit, by offering it via a new option:
> `--dual-color`.
>
> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>  diff.c | 83 +++++++++++++++++++++++++++++++++++++++++++++++-----------
>  diff.h |  1 +
>  2 files changed, 69 insertions(+), 15 deletions(-)
>
> diff --git a/diff.c b/diff.c
> index a94a8214f..e163bc8a3 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -563,14 +563,18 @@ static void check_blank_at_eof(mmfile_t *mf1, mmfile_t *mf2,
>  	ecbdata->blank_at_eof_in_postimage = (at - l2) + 1;
>  }
>  
> -static void emit_line_0(struct diff_options *o, const char *set, const char *reset,
> +static void emit_line_0(struct diff_options *o,
> +			const char *set, unsigned reverse, const char *reset,
>  			int first, const char *line, int len)
>  {
>  	int has_trailing_newline, has_trailing_carriage_return;
>  	int nofirst;
>  	FILE *file = o->file;
>  
> -	fputs(diff_line_prefix(o), file);
> +	if (first)
> +		fputs(diff_line_prefix(o), file);
> +	else if (!len)
> +		return;