Expand ~ to user's home directory for --reference

This allows a user to have a 'repo init' as:
  $ repo init -u ... --reference=~/mirror

Change-Id: Ib85b7c8ffca9d732132c68fe9a8d7f0ab1fa9288

subcmds/init.py

@@ -317,6 +317,10 @@ to update the working directory files.
 
   def Execute(self, opt, args):
     git_require(MIN_GIT_VERSION, fail=True)
+
+    if opt.reference:
+      opt.reference = os.path.expanduser(opt.reference)
+
     self._SyncManifest(opt)
     self._LinkManifest(opt.manifest_name)