Add logging system to pub and sprinkle some logging in.

utils/pub/git.dart

Index: utils/pub/git.dart
diff --git a/utils/pub/git.dart b/utils/pub/git.dart
index 80901a98fef5762c165178461291dd6a6ab4049b..6defde309a90d0730f614f88a2f3e0d26087297a 100644
--- a/utils/pub/git.dart
+++ b/utils/pub/git.dart
@@ -8,6 +8,7 @@
 library git;
 
 import 'io.dart';
+import 'log.dart' as log;
 import 'utils.dart';
 
 /// Tests whether or not the git command-line app is available for use.
@@ -57,6 +58,7 @@ Future<String> get _gitCommand {
       return null;
     });
   }).transform((command) {
+    log.fine('Determined git command $command.');
     _gitCommandCache = command;
     return command;
   });