cmd/dist: restore behavior of branchtag

The behavior was changed unintentionally during the conversion from C to Go.
Reviewed-on: https://go-review.googlesource.com/2470

This belongs to a series of clean-up changes (see below) for cmd/dist.
This is change (2).

These changes include:
(1)  apply minor fixes
(2)  restore behavior of branchtag
(3)  unleash bootstrap optimization for windows
(4)  use standard generated code header
(5)  remove trivial variables + functions
(6)  move functions for the better
(7)  simplify code segments
(8)  use bytes.Buffer for code generation
(9)  rename variables + functions
(10) remove doc.go

Change-Id: I53373c7211b35ed68da485c55e510871bfb81267
Reviewed-on: https://go-review.googlesource.com/61022Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/build.go

@@ -253,7 +253,7 @@ func chomp(s string) string {
 func branchtag(branch string) (tag string, precise bool) {
 	b := run(goroot, CheckExit, "git", "log", "--decorate=full", "--format=format:%d", "master.."+branch)
 	tag = branch
-	for _, line := range splitlines(b) {
+	for row, line := range splitlines(b) {
 		// Each line is either blank, or looks like
 		//	  (tag: refs/tags/go1.4rc2, refs/remotes/origin/release-branch.go1.4, refs/heads/release-branch.go1.4)
 		// We need to find an element starting with refs/tags/.
@@ -271,7 +271,7 @@ func branchtag(branch string) (tag string, precise bool) {
 			continue // malformed line; ignore it
 		}
 		tag = line[i : i+j]
-		if i == 0 {
+		if row == 0 {
 			precise = true // tag denotes HEAD
 		}
 		break