git: revert c947bf808 -- it triggers a bug.

We seem to have a botch in the protocol negotiation, where
we leak some protocol packets into the packfile; this will
need to be fixed before we put this change in.

sys/src/cmd/git/fetch.c

@@ -260,7 +260,7 @@ fetchpack(Conn *c)
 	if(writepkt(c, buf, n) == -1)
 		sysfatal("write: %r");
 	if(!req)
-		goto showrefs;
+		return 0;
 	if(readphase(c) == -1)
 		sysfatal("read: %r");
 	if((n = readpkt(c, buf, sizeof(buf))) == -1)

sys/src/cmd/git/pull

@@ -7,10 +7,13 @@ fn update{
 	upstream=$2
 	url=$3
 	dir=$4
+	bflag=()
 	dflag=()
+	if(! ~ $#branch 0)
+		bflag=(-b $branch)
 	if(! ~ $#debug 0)
 		dflag='-d'
-	{git/fetch $dflag -u $upstream $url >[2=3] || die $status} | awk '
+	{git/fetch $dflag $bflag -u $upstream $url >[2=3] || die $status} | awk '
 	/^remote/{
 		if($2=="HEAD")
 			next