Add cipd selfupdate command.

cipd/client/cipd/client_test.go

Index: cipd/client/cipd/client_test.go
diff --git a/cipd/client/cipd/client_test.go b/cipd/client/cipd/client_test.go
index 2261c636ee418d875b4ae35fe6c1f17efb7c73ab..6df3f3f4ca03a1cc8e8e4b2fc94ed7256577a814 100644
--- a/cipd/client/cipd/client_test.go
+++ b/cipd/client/cipd/client_test.go
@@ -24,6 +24,7 @@ import (
 	"github.com/luci/luci-go/common/logging/gologger"
 
 	"github.com/luci/luci-go/cipd/client/cipd/common"
+	"github.com/luci/luci-go/cipd/client/cipd/internal"
 	"github.com/luci/luci-go/cipd/client/cipd/local"
 
 	. "github.com/smartystreets/goconvey/convey"
@@ -527,6 +528,22 @@ func TestFetch(t *testing.T) {
 	})
 }
 
+func TestMaybeUpdateClient(t *testing.T) {
+	ctx := makeTestContext()
+
+	Convey("MaybeUpdateClient", t, func() {
+
+		Convey("Is a NOOP when exeHash matches", func(c C) {
+			client := mockClient(c, "", nil)
+			client.tagCache = internal.NewTagCache(nil)
+			pin := common.Pin{clientPackage, "0000000000000000000000000000000000000000"}
+			So(client.tagCache.AddTag(ctx, pin, "git:deadbeef"), ShouldBeNil)
+			So(client.tagCache.AddFile(ctx, pin, clientFileName, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"), ShouldBeNil)
+			So(client.MaybeUpdateClient(ctx, nil, "git:deadbeef", "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "some_path"), ShouldBeNil)
+		})
+	})
+}
+
 func TestProcessEnsureFile(t *testing.T) {
 	ctx := makeTestContext()