[9front] git: tune deltification

[ori@eigenstate.org]

Dropping the chunk size reduces pack sizes
by about 15%, from 120 megs to 100.

Replacing sha1 with murmurhash3 when hashing
deltas drops the time to repack the 9front
repo by about 20 seconds.

diff 2367a2aeaec8432e6b059135e49c2fa86e415ae5 uncommitted
--- a/sys/src/cmd/git/delta.c
+++ b/sys/src/cmd/git/delta.c
@@ -1,13 +1,13 @@
 #include <u.h>
 #include <libc.h>
+#include <fcall.h>
 
 #include "git.h"
 
 enum {
-	Minchunk	= 128,
+	Minchunk	= 32,
+	Splitmask	= 0x7f,
 	Maxchunk	= 8192,
-	Splitmask	= (1<<8)-1,
-	
 };
 
 static u32int geartab[] = {
@@ -45,12 +45,43 @@
     0x9984a4f4, 0xd5de43cc, 0xd294daed, 0xbecba2d2, 0xf1f6e72c, 0x5551128a, 0x83af87e2, 0x6f0342ba,
 };
 
-static u64int
-hash(void *p, int n)
+/* murmurhash3 */
+u32int
+hash(void *ptr, int len)
 {
-	uchar buf[SHA1dlen];
-	sha1((uchar*)p, n, buf, nil);
-	return GETBE64(buf);
+	u32int h, k, s;
+	uchar *p;
+	int i;
+
+	/* Read in groups of 4. */
+	h = 2928213749ul;
+	p = ptr;
+	for (i = len >> 2; i; i--) {
+		k = *(u32int*)p;
+		s = k * 0xcc9e2d51;
+		s = (s << 15) | (s >> 17);
+		h ^= s*0x1b873593;
+		h = (h << 13) | (h >> 19);
+		h = h * 5 + 0xe6546b64;
+		p += 4;
+	}
+	/* Read the rest. */
+	k = 0;
+	for (i = len & 3; i; i--) {
+		k <<= 8;
+		k |= p[i - 1];
+	}
+	s = k * 0xcc9e2d51;
+	s = (s << 15) | (s >> 17);
+	h ^= s*0x1b873593;
+	/* Finalize. */
+	h ^= len;
+	h ^= h >> 16;
+	h *= 0x85ebca6b;
+	h ^= h >> 13;
+	h *= 0xc2b2ae35;
+	h ^= h >> 16;
+	return h;
 }
 
 static void