go/build: add go1.13 release tag

Adding this early in the cycle to start regression testing in the master
toolchain.

Change-Id: Ia151429c4f94efbac0aa41ab6bc16e7462b0e303
Reviewed-on: https://go-review.googlesource.com/c/163082
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/build/build.go

@@ -299,7 +299,7 @@ func defaultContext() Context {
 	// (perhaps it is the stub to use in that case) should say "+build !go1.x".
 	// NOTE: If you add to this list, also update the doc comment in doc.go.
 	// NOTE: The last element in ReleaseTags should be the current release.
-	const version = 12 // go1.12
+	const version = 13 // go1.13
 	for i := 1; i <= version; i++ {
 		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
 	}

src/go/build/doc.go

@@ -109,6 +109,7 @@
 //	- "go1.10", from Go version 1.10 onward
 //	- "go1.11", from Go version 1.11 onward
 //	- "go1.12", from Go version 1.12 onward
+//	- "go1.13", from Go version 1.13 onward
 //	- any additional words listed in ctxt.BuildTags
 //
 // There are no build tags for beta or minor releases.