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

src/borg/_hashindex.c

@@ -146,7 +146,10 @@ hashindex_lookup(HashIndex *index, const void *key, int *start_idx)
             }
             return idx;
         }
-        idx = (idx + 1) % index->num_buckets;
+        idx++;
+        if (idx >= index->num_buckets) {
+            idx -= index->num_buckets;
+        }
         if(idx == start) {
             break;
         }
@@ -547,7 +550,10 @@ hashindex_set(HashIndex *index, const void *key, const void *value)
         }
         idx = start_idx;
         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--;