Added rbyd < 1/4 block_size condition to btree merges

This just avoids the overhead of estimating our sibling's sizes, which
we don't really want to do every compact.

lfs.c

@@ -3946,23 +3946,27 @@ static int lfsr_btree_commit(lfs_t *lfs, lfsr_btree_t *btree,
 
         // before we compact, can we merge with our siblings?
         lfsr_rbyd_t sibling;
+        if ((lfs_size_t)estimate <= lfs->cfg->block_size/4
+                // no parent? can't merge
+                && rid != -1) {
             for (uint8_t i = 0; i < 2; i++) {
                 lfs_ssize_t sibling_rid;
                 // try the right sibling
                 if (i == 0) {
                     sibling_rid = rid+1;
+                    // no right sibling? can't merge
+                    if (sibling_rid >= (lfs_ssize_t)parent.weight) {
+                        continue;
+                    }
 
                 // try the left sibling
                 } else {
                     sibling_rid = rid-rbyd.weight;
-            }
+                    // no left sibling? can't merge
-
+                    if (sibling_rid < 0) {
-            // no parent? no sibling?
-            if (rid == -1
-                    || sibling_rid < 0
-                    || sibling_rid >= (lfs_ssize_t)parent.weight) {
                         continue;
                     }
+                }
 
                 // try looking up the sibling
                 // TODO do we really need to fetch sibling_weight if we get
@@ -4023,6 +4027,7 @@ static int lfsr_btree_commit(lfs_t *lfs, lfsr_btree_t *btree,
                     goto merge;
                 }
             }
+        }
 
         // allocate a new rbyd
         err = lfsr_rbyd_alloc(lfs, &rbyd_);