Re: [PATCH 01/14] strbuf: introduce strbuf_prefixf()

> When preparing an error message in a strbuf, it can be convenient
> to add a formatted string to the beginning:
>
>         if (transaction_commit(&t, err)) {
>                 strbuf_prefixf(err, "cannot fetch '%s': ", remotename);
>                 return -1;
>         }
>
> The new strbuf_prefixf is like strbuf_addf, except it writes its
> result to the beginning of a strbuf instead of the end.
>
> The current implementation uses strlen(strfmt(fmt, ...)) extra bytes
> at the end of the strbuf as temporary scratch space for convenience
> and simplicity.  A later patch can optimize if needed.
>
> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
> ---
> diff --git a/strbuf.c b/strbuf.c
> index 0346e74..3f4aaa3 100644
> --- a/strbuf.c
> +++ b/strbuf.c
> @@ -219,6 +219,22 @@ void strbuf_addf(struct strbuf *sb, const char *fmt, ...)
>         va_end(ap);
>  }
>
> +void strbuf_prefixf(struct strbuf *sb, const char *fmt, ...)
> +{
> +       va_list ap;
> +       size_t pos, len;
> +
> +       pos = sb->len;
> +
> +       va_start(ap, fmt);
> +       strbuf_vaddf(sb, fmt, ap);
> +       va_end(ap);
> +
> +       len = sb->len - pos;
> +       strbuf_insert(sb, 0, sb->buf + pos, len);
> +       strbuf_remove(sb, pos + len, len);

> +}
> +
>  static void add_lines(struct strbuf *out,
>                         const char *prefix1,
>                         const char *prefix2,