query: simplify empty BranchesRepos and RepoIDs

We recently had an incident where this was accidently unset and lead to
lots of work done by zoekt which just got thrown away.

Test Plan: added unit test

query/query.go

@@ -651,6 +651,17 @@ func evalConstants(q Q) Q {
 		if s.Pattern == "" {
 			return &Const{true}
 		}
+	case *BranchesRepos:
+		for _, br := range s.List {
+			if !br.Repos.IsEmpty() {
+				return q
+			}
+		}
+		return &Const{false}
+	case *RepoIDs:
+		if s.Repos.IsEmpty() {
+			return &Const{false}
+		}
 	case *RepoSet:
 		if len(s.Set) == 0 {
 			return &Const{false}

query/query_test.go

@@ -76,6 +76,20 @@ func TestSimplify(t *testing.T) {
 				&Substring{Pattern: "byte"},
 				&Not{&Substring{Pattern: "byte"}}),
 		},
+		{
+			in: NewAnd(
+				NewSingleBranchesRepos("HEAD"), // Empty list matches nothing
+				&Not{&Type{Type: TypeRepo, Child: &Substring{Pattern: "hi"}}}),
+			want: &Const{false},
+		},
+		{
+			in: NewAnd(
+				NewSingleBranchesRepos("HEAD", 1),
+				&Not{&Type{Type: TypeRepo, Child: &Substring{Pattern: "hi"}}}),
+			want: NewAnd(
+				NewSingleBranchesRepos("HEAD", 1),
+				&Not{&Type{Type: TypeRepo, Child: &Substring{Pattern: "hi"}}}),
+		},
 	}
 
 	for _, c := range cases {