RepoStats: set default shard count to 1

Currently, Zoekt always reports 0 shards even if a repository has more
than 1 shard.

The reason is that `shardedSeacher.List` aggregates stats by calling
`Stats.Add` while looping over shards. However, since shards have a
default shard count of 0, the shard count never changes.

Change-Id: I06ca00952240a21cd27581055169683366562be8
diff --git a/index_test.go b/index_test.go
index d731444..c8f987f 100644
--- a/index_test.go
+++ b/index_test.go
@@ -1087,6 +1087,9 @@
 	if len(res.Repos) != 1 || res.Repos[0].Repository.Name != "reponame" {
 		t.Fatalf("got %v, want 1 matches", res)
 	}
+	if got := res.Repos[0].Stats.Shards; got != 1 {
+		t.Fatalf("got %d, want 1 shard", got)
+	}
 	q = &query.Repo{Pattern: "bla"}
 	res, err = searcher.List(context.Background(), q)
 	if err != nil {
diff --git a/indexdata.go b/indexdata.go
index 6c02cd5..7072951 100644
--- a/indexdata.go
+++ b/indexdata.go
@@ -104,6 +104,7 @@
 		IndexBytes:   int64(d.memoryUse()),
 		ContentBytes: int64(int(last) + int(lastFN)),
 		Documents:    len(d.newlinesIndex) - 1,
+		Shards:       1,
 	}
 	d.repoListEntry = RepoListEntry{
 		Repository:    d.repoMetaData,