Completion for build-level keys

sbt's shell provided completion only for keys that were relative to a
defined project, but didn't provide completion for keys that belong to
the build definition only.

This commit fixes this issue by defining a new kind of `Parser` (from
which completions are generated) which runs its input simultaneously on
distinct parsers. We now define a parser for project-level keys and
another parser for build-level keys. These two parsers are eventually
combined, and we get the completions of both parsers.

Fixes sbt/sbt#2460

main/src/main/scala/sbt/internal/Act.scala

@@ -49,12 +49,22 @@ object Act {
           new ParsedKey(makeScopedKey(proj, conf, task, extra, key), mask)
         }
 
-      for {
-        rawProject <- optProjectRef(index, current)
-        proj = resolveProject(rawProject, current)
-        confAmb <- config(index configs proj)
-        partialMask = ScopeMask(rawProject.isExplicit, confAmb.isExplicit, false, false)
-      } yield taskKeyExtra(proj, confAmb, partialMask)
+      val projectKeys =
+        for {
+          rawProject <- optProjectRef(index, current)
+          proj = resolveProject(rawProject, current)
+          confAmb <- config(index configs proj)
+          partialMask = ScopeMask(rawProject.isExplicit, confAmb.isExplicit, false, false)
+        } yield taskKeyExtra(proj, confAmb, partialMask)
+
+      val build = Some(BuildRef(current.build))
+      val buildKeys =
+        for {
+          confAmb <- config(index configs build)
+          partialMask = ScopeMask(false, confAmb.isExplicit, false, false)
+        } yield taskKeyExtra(build, confAmb, partialMask)
+
+      buildKeys combinedWith projectKeys map (_.flatten)
     }
   def makeScopedKey(proj: Option[ResolvedReference], conf: Option[String], task: Option[AttributeKey[_]], extra: ScopeAxis[AttributeMap], key: AttributeKey[_]): ScopedKey[_] =
     ScopedKey(Scope(toAxis(proj, Global), toAxis(conf map ConfigKey.apply, Global), toAxis(task, Global), extra), key)