replace modulo with if to check for wraparound in hashmap

Integer division is slow, and this improves the speed of all operations on the hashmap.

Benchmarked this patch on the rciorba/master-bench branch:
https://cdn.rawgit.com/rciorba/e3eded290e91a361f74eefdbc84416bb/raw/9e5d61e03c5842712d629e3e5567da3a512f1d79/results.html

(cherry picked from commit 12e0f559912093fdf26366d48a51f99e7a97d80f)

borg/_hashindex.c

@@ -143,7 +143,10 @@ hashindex_lookup(HashIndex *index, const void *key)
             }
             return idx;
         }
-        idx = (idx + 1) % index->num_buckets;
+        idx++;
+        if (idx >= index->num_buckets) {
+            idx -= index->num_buckets;
+        }
         if(idx == start) {
             return -1;
         }
@@ -434,7 +437,10 @@ hashindex_set(HashIndex *index, const void *key, const void *value)
         }
         idx = hashindex_index(index, key);
         while(!BUCKET_IS_EMPTY(index, idx) && !BUCKET_IS_DELETED(index, idx)) {
-            idx = (idx + 1) % index->num_buckets;
+            idx++;
+            if (idx >= index->num_buckets){
+                idx -= index->num_buckets;
+            }
         }
         if(BUCKET_IS_EMPTY(index, idx)){
             index->num_empty--;