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packfiles: fix offset deltas

Git has yet another integer encoding for offset deltas,
and we need to use it.

--- a/pack.c

+++ b/pack.c

@@ -399,19 +399,19 @@

 	Object b;

 	char *d;

 	vlong r;

-	int c, s, n;

+	int c, n;

 	d = nil;

-	r = 0;

-	s = 0;

-	do {

+	if((c = Bgetc(f)) == -1)

+		return -1;

+	r = c & 0x7f;

+	while(c & 0x80 && r < (1ULL<<56)){

 		if((c = Bgetc(f)) == -1)

 			return -1;

-		r |= (c & 0x7f) << s;

-		s += 7;

-	} while (c & 0x80 && s < 63);

+		r = ((r + 1)<<7) | (c & 0x7f);

+	}

-	if(r > p || s > 63){

+	if(r > p || r >= (1ULL<<56)){

 		werrstr("junk offset -%lld (from %lld)", r, p);

 		goto error;

 	}

@@ -1472,18 +1472,17 @@

 static int

 packoff(char *hdr, vlong off)

 {

-	int i;

+	int i, j;

+	char rbuf[8];

-	hdr[0] = off & 0x7f;

-	off >>= 7;

-	for(i = 1; off != 0; i++){

-		assert(i < sizeof(hdr));

-		hdr[i-1] |= 0x80;

-		hdr[i] = off & 0x7f;

-		off >>= 7;

-	}

-	return i;

+	rbuf[0] = off & 0x7f;

+	for(i = 1; (off >>= 7) != 0; i++)

+		rbuf[i] |= (--off & 0x7f)|0x80;

+	j = 0;

+	while(i > 0)

+		hdr[j++] = rbuf[--i];

+	return j;

 }

 static int