sam: add $% as an alias for $samfile

R=r
http://codereview.appspot.com/6488137
diff --git a/src/cmd/sam/shell.c b/src/cmd/sam/shell.c
index 32e1b3d..c6efdd5 100644
--- a/src/cmd/sam/shell.c
+++ b/src/cmd/sam/shell.c
@@ -17,6 +17,7 @@
 	else
 		buf[0] = 0;
 	putenv("samfile", buf);
+	putenv("%", buf); // like acme
 }
 
 int