Fix diff -B/--dirstat miscounting of newly added contents

What used to happen is that diffcore_count_changes() simply ignored any
hashes in the destination that didn't match hashes in the source. EXCEPT
if the source hash didn't exist at all, in which case it would count _one_
destination hash that happened to have the "next" hash value.  As a
consequence, newly added material was often undercounted, making output
from --dirstat and "complete rewrite" detection used by -B unrelialble.

This changes it so that:

 - whenever it bypasses a destination hash (because it doesn't match a
   source), it counts the bytes associated with that as "literal added"

 - at the end (once we have used up all the source hashes), we do the same
   thing with the remaining destination hashes.

 - when hashes do match, and we use the difference in counts as a value,
   we also use up that destination hash entry (the 'd++').

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-delta.c

@@ -201,10 +201,15 @@ int diffcore_count_changes(struct diff_filespec *src,
 		while (d->cnt) {
 			if (d->hashval >= s->hashval)
 				break;
+			la += d->cnt;
 			d++;
 		}
 		src_cnt = s->cnt;
-		dst_cnt = d->hashval == s->hashval ? d->cnt : 0;
+		dst_cnt = 0;
+		if (d->cnt && d->hashval == s->hashval) {
+			dst_cnt = d->cnt;
+			d++;
+		}
 		if (src_cnt < dst_cnt) {
 			la += dst_cnt - src_cnt;
 			sc += src_cnt;
@@ -213,6 +218,10 @@ int diffcore_count_changes(struct diff_filespec *src,
 			sc += dst_cnt;
 		s++;
 	}
+	while (d->cnt) {
+		la += d->cnt;
+		d++;
+	}
 
 	if (!src_count_p)
 		free(src_count);