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packfiles: fix indexing large pack files (thanks Igor Boehm)

This fixes two bugs with packfiles larger than 2
gigs in size.

When writing out the index, we were incremeneting
the index of the 64 bit offset multiple times due
to macro expansion; this patch changes it so that
we only increment it once per iteration.

When reading the index, we were interpreting the
large offset index as the offset of the 64 bit
offset, leading to a garbled index. This diff also
fixes our interpretation of the packfile, so we
read all objects correctly, even when they're at
a large offset.

--- a/pack.c

+++ b/pack.c

@@ -673,11 +673,21 @@

 		goto err;

 	i = GETBE32(idx + oo);

 	o = i & 0xffffffffULL;

+	/*

+	 * Large offsets (i.e. 64-bit) are encoded as an index

+	 * into the next table with the MSB bit set.

+	 */

 	if(o & (1ull << 31)){

-		o &= 0x7fffffff;

-		if(o < 0 || o + 8 >= nidx)

+		o &= 0x7fffffffULL;

+		oo = 8;				/* Header */

+		oo += 256*4;			/* Fanout table */

+		oo += Hashsz*nent;		/* Hashes */

+		oo += 4*nent;			/* Checksums */

+		oo += 4*nent;			/* 32-bit Offsets */

+		oo += 8*o;			/* 64-bit Offset offset */

+		if(oo < 0 || oo + 8 >= nidx)

 			goto err;

-		o = GETBE64(idx + o);

+		o = GETBE64(idx + oo);

 	}

 	return o;

@@ -1146,14 +1156,16 @@

 	nbig = 0;

 	for(i = 0; i < nobj; i++){

-		if(obj[i]->off <= (1ull<<31))

+		if(obj[i]->off < (1ull<<31))

 			PUTBE32(buf, obj[i]->off);

-		else

-			PUTBE32(buf, (1ull << 31) | nbig++);

+		else{

+			PUTBE32(buf, (1ull << 31) | nbig);

+			nbig++;

+		}

 		hwrite(f, buf, 4, &st);

 	}

 	for(i = 0; i < nobj; i++){

-		if(obj[i]->off > (1ull<<31)){

+		if(obj[i]->off >= (1ull<<31)){

 			PUTBE64(buf, obj[i]->off);

 			hwrite(f, buf, 8, &st);

 		}

--- a/save.c

+++ b/save.c

@@ -254,6 +254,8 @@

 		e = dirent(&ent, &nent, elt);

 		if(e->islink)

 			sysfatal("symlinks may not be modified: %s", *path);

+		if(e->ismod)

+			sysfatal("submodules may not be modified: %s", *path);

 		if(isdir){

 			e->mode = DMDIR | 0755;

 			sub[nsub] = readobject(e->h);