goinstall: allow packages from launchpad.net/~user branches.

The permitted filename characters should include ~ to allow
the names of user-owned branches in Launchpad.

R=golang-dev, rsc, n13m3y3r, gustavo
CC=golang-dev, gustavo.niemeyer
https://golang.org/cl/5280052

src/cmd/goinstall/make.go

@@ -109,7 +109,7 @@ func makeMakefile(dir, pkg string, tree *build.Tree, isCmd bool) ([]byte, error)
 	return buf.Bytes(), nil
 }
 
-var safeBytes = []byte("+-./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz")
+var safeBytes = []byte("+-~./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz")
 
 func safeName(s string) bool {
 	if s == "" {
@@ -118,6 +118,9 @@ func safeName(s string) bool {
 	if strings.Contains(s, "..") {
 		return false
 	}
+	if s[0] == '~' {
+		return false
+	}
 	for i := 0; i < len(s); i++ {
 		if c := s[i]; c < 0x80 && bytes.IndexByte(safeBytes, c) < 0 {
 			return false