GitCommit: Let us specify the date time

The Timezone handling is java seems to be quite strange, so I'm going to
skip it for now and fix it later. We do seem to have proper timezone
classes in future versions of Java, but that requires me to increase the
minSDKVersion, and therefore not support very old android devices.

lib/gitapp.dart

@@ -40,7 +40,7 @@ buildGitButtons() {
     RaisedButton(
       child: Text("Git Add"),
       onPressed: () async {
-        await gitAdd("journal", "1");
+        await gitAdd("journal", ".");
       },
     ),
     RaisedButton(
@@ -57,6 +57,7 @@ buildGitButtons() {
             authorEmail: "noemail@example.com",
             authorName: "Vishesh Handa",
             message: "Default message from GitJournal",
+            when: "2017-10-20T01:21:10+02:00",
           );
         }),
   ];