Indent timing logs

Sbt has a feature to show timed logs for every operation at startup.
However, its output is cluttered and users cannot read how much time
single methods consume nor if they call other methods.

This commit improves the status quo by adding indentation.

main/src/main/scala/sbt/internal/Load.scala

@@ -1015,12 +1015,18 @@ private[sbt] object Load {
       }
   }
 
+  /** Variable to control the indentation of the timing logs. */
+  private var timedIndentation: Int = 0
+
   /** Debugging method to time how long it takes to run various compilation tasks. */
   private[sbt] def timed[T](label: String, log: Logger)(t: => T): T = {
+    timedIndentation += 1
     val start = System.nanoTime
     val result = t
     val elapsed = System.nanoTime - start
-    log.debug(label + " took " + (elapsed / 1e6) + " ms")
+    timedIndentation -= 1
+    val prefix = " " * 2 * timedIndentation
+    log.debug(s"$prefix$label took ${elapsed / 1e6}ms")
     result
   }
 }