[9front] git: only send object ids once

[ori@eigenstate.org]

When fetching, we could send some object ids twice,
if the same branch was pointing at the same object.

This appears to tickle some issue in upstream git,
making it fail to respect the fetch protocol, and
sending multiple ack lines where it should send one.

this change adds a set of objects we said we have,
and we only send if we haven't already sent it.

later, we will implement multi_ack.

diff facb0e757ac63f763bd942a2714f979538b99eb0 uncommitted
--- a/sys/src/cmd/git/fetch.c
+++ b/sys/src/cmd/git/fetch.c
@@ -186,6 +186,7 @@
 	int nref, refsz, first;
 	int i, n, req, pfd;
 	vlong packsz;
+	Objset hadobj;
 	Object *o;
 
 	nref = 0;
@@ -246,13 +247,19 @@
 		req = 1;
 	}
 	flushpkt(c);
+	osinit(&hadobj);
 	for(i = 0; i < nref; i++){
-		if(hasheq(&have[i], &Zhash))
+		if(hasheq(&have[i], &Zhash) || oshas(&hadobj, have[i]))
 			continue;
+		if((o = readobject(have[i])) == nil)
+			sysfatal("missing object we should have: %H", have[i]);
+		osadd(&hadobj, o);
+		unref(o);	
 		n = snprint(buf, sizeof(buf), "have %H\n", have[i]);
 		if(writepkt(c, buf, n + 1) == -1)
 			sysfatal("could not send have for %H", have[i]);
 	}
+	osclear(&hadobj);
 	if(!req)
 		flushpkt(c);