Re: [PATCH v13 02/15] path.c: refactor relative_path(), not only strip prefix

> Original design of relative_path() is simple, just strip the prefix
> (*base) from the absolute path (*abs). In most cases, we need a real
> relative path, such as: ../foo, ../../bar. That's why there is another
> reimplementation (path_relative()) in quote.c.
> 
> Refactor relative_path() in path.c to return real relative path, so
> that user can reuse this function without reimplement his/her own.
> I will use this method for interactive git-clean later. Some of the
> implementations are borrowed from path_relative() in quote.c.
> 
> Different results for relative_path() before and after this refactor:
> 
>     abs path  base path  relative (original)  relative (refactor)
>     ========  =========  ===================  ===================
>     /a/b/c/   /a/b       c/                   c/
>     /a/b//c/  //a///b/   c/                   c/
>     /a/b      /a/b       .                    ./
>     /a/b/     /a/b       .                    ./
>     /a        /a/b/      /a                   ../
>     /         /a/b/      /                    ../../
>     /a/c      /a/b/      /a/c                 ../c
>     /a/b      (empty)    /a/b                 /a/b
>     /a/b      (null)     /a/b                 /a/b
>     (empty)   /a/b       (empty)              ./
>     (null)    (empty)    (null)               ./
>     (null)    /a/b       (segfault)           ./

> diff --git a/path.c b/path.c
> index 04ff..0174d 100644
> --- a/path.c
> +++ b/path.c
> @@ -441,42 +441,104 @@ int adjust_shared_perm(const char *path)
>  	return 0;
>  }
>  
> -const char *relative_path(const char *abs, const char *base)
> +/*
> + * Give path as relative to prefix.
> + *
> + * The strbuf may or may not be used, so do not assume it contains the
> + * returned path.
> + */
> +const char *relative_path(const char *abs, const char *base,
> +			  struct strbuf *sb)