rerere: modernize use of empty strbuf

Back when the code in the handle_conflict() helper function that
hashes the contents stored in the strbuf "one" and "two", including
its terminating NUL, was written, a freshly initialized strbuf had
NULL in its .buf member, so it was an error to say

    update(one.buf, one.len + 1)

which was corrected by b4833a2c (rerere: Fix use of an empty
strbuf.buf, 2007-09-26).

But soon after that, b315c5c0 (strbuf change: be sure ->buf is never
ever NULL., 2007-09-27) introduced strbuf_slopbuf mechanism that
ensures that .buf member of a strbuf is *never* NULL.  A freshly
initialized and empty strbuf uses a static piece of memory that has
NUL in it, with its .len member set to 0, so we can always safely
use from offset 0 of .buf[] array for (one.len + 1) bytes.

Simplify the code by essentially reverting the b4833a2c, whose fix
is no longer necessary in the modern world order.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.c

@@ -390,12 +390,8 @@ static int handle_conflict(struct strbuf *out, struct rerere_io *io,
 			strbuf_addbuf(out, &two);
 			rerere_strbuf_putconflict(out, '>', marker_size);
 			if (ctx) {
-				the_hash_algo->update_fn(ctx, one.buf ?
-							 one.buf : "",
-							 one.len + 1);
-				the_hash_algo->update_fn(ctx, two.buf ?
-							 two.buf : "",
-							 two.len + 1);
+				the_hash_algo->update_fn(ctx, one.buf, one.len + 1);
+				the_hash_algo->update_fn(ctx, two.buf, two.len + 1);
 			}
 			break;
 		} else if (hunk == RR_SIDE_1)