[release-branch.go1.10-security] cmd/go/internal/get: use a strings.Replacer in expand

This should be a no-op, but produces deterministic (and more correct)
behavior if we have accidentally failed to sanitize one of the inputs.

Change-Id: I1271d0ffd01a691ec8c84906c4e02d9e2be19c72
Reviewed-on: https://team-review.git.corp.google.com/c/372707
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills

src/cmd/go/internal/get/vcs.go

@@ -970,10 +970,14 @@ func matchGoImport(imports []metaImport, importPath string) (metaImport, error)
 
 // expand rewrites s to replace {k} with match[k] for each key k in match.
 func expand(match map[string]string, s string) string {
+	// We want to replace each match exactly once, and the result of expansion
+	// must not depend on the iteration order through the map.
+	// A strings.Replacer has exactly the properties we're looking for.
+	oldNew := make([]string, 0, 2*len(match))
 	for k, v := range match {
-		s = strings.Replace(s, "{"+k+"}", v, -1)
+		oldNew = append(oldNew, "{"+k+"}", v)
 	}
-	return s
+	return strings.NewReplacer(oldNew...).Replace(s)
 }
 
 // vcsPaths defines the meaning of import paths referring to