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deltify: limit delta chain length.

We don't want insanely long delta chains, for a number
of reasons. Upstream git limits them to 4095, we should
do the same.

--- a/pack.c

+++ b/pack.c

@@ -9,16 +9,19 @@

 typedef struct Compout	Compout;

 struct Objmeta {

+	/* For sorting */

 	int	type;

 	char	*path;

 	vlong	mtime;

 	Hash	hash;

-	Dtab	tab;

+	/* The best delta we picked */

 	Object	*obj;

 	Object	*base;

 	Delta	*delta;

 	int	ndelta;

+	int	nchain;

+

 	Dtab 	dtab;

 };

@@ -1282,16 +1285,25 @@

 		m->ndelta = 0;

 		dtinit(&m->dtab, a->data, a->size);

 		for(; p != m; p++){

+			/* lets not make the chains too long */

+			if(p->nchain >= 4095)

+				continue;

 			if((b = readobject(p->hash)) == nil)

 				sysfatal("missing object %H", p->hash);

 			d = deltify(a->data, a->size, &p->dtab, &nd);

 			sz = deltasz(d, nd);

 			if(sz + 32 < best){

+				/*

+				 * if we already picked a best delta,

+				 * replace it.

+				 */

 				free(m->delta);

 				best = sz;

 				m->base = b;

 				m->delta = d;

 				m->ndelta = nd;

+				m->nchain = p->nchain + 1;

+					

 			}else

 				free(d);

 			unref(b);