Re: [PATCH v4] diff: handle --no-abbrev in no-index case

> There are two different places where the --no-abbrev option is parsed,
> and two different places where SHA-1s are abbreviated. We normally parse
> --no-abbrev with setup_revisions(), but in the no-index case, "git diff"
> calls diff_opt_parse() directly, and diff_opt_parse() didn't handle
> --no-abbrev until now. (It did handle --abbrev, however.) We normally
> abbreviate SHA-1s with find_unique_abbrev(), but commit 4f03666 ("diff:
> handle sha1 abbreviations outside of repository, 2016-10-20) recently
> introduced a special case when you run "git diff" outside of a
> repository.
>
> setup_revisions() does also call diff_opt_parse(), but not for --abbrev
> or --no-abbrev, which it handles itself. setup_revisions() sets
> rev_info->abbrev, and later copies that to diff_options->abbrev. It
> handles --no-abbrev by setting abbrev to zero. (This change doesn't
> touch that.)
>
> Setting abbrev to zero was broken in the outside-of-a-repository special
> case, which until now resulted in a truly zero-length SHA-1, rather than
> taking zero to mean do not abbreviate. The only way to trigger this bug,
> however, was by running "git diff --raw" without either the --abbrev or
> --no-abbrev options, because 1) without --raw it doesn't respect abbrev
> (which is bizarre, but has been that way forever), 2) we silently clamp
> --abbrev=0 to MINIMUM_ABBREV, and 3) --no-abbrev wasn't handled until
> now.
>
> The outside-of-a-repository case is one of three no-index cases. The
> other two are when one of the files you're comparing is outside of the
> repository you're in, and the --no-index option.

> diff --git a/diff.c b/diff.c
> index ec87283..84dba60 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -3106,7 +3106,8 @@ static const char *diff_abbrev_oid(const struct object_id *oid, int abbrev)
>  			abbrev = FALLBACK_DEFAULT_ABBREV;
>  		if (abbrev > GIT_SHA1_HEXSZ)
>  			die("BUG: oid abbreviation out of range: %d", abbrev);
> -		hex[abbrev] = '\0';
> +		if (abbrev)
> +			hex[abbrev] = '\0';
>  		return hex;
>  	}
>  }

> @@ -3364,6 +3365,7 @@ void diff_setup(struct diff_options *options)
>  
>  	options->file = stdout;
>  
> +	options->abbrev = DEFAULT_ABBREV;