Avoid BugSense reported IllegalStateException.

java.lang.IllegalStateException: Recursive entry to executePendingTransactions
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1388)
[...]
at android.database.DataSetObservable.notifyChanged(DataSetObservable.java:37)
at android.support.v4.view.PagerAdapter.notifyDataSetChanged(PagerAdapter.java:276)
at com.gh4a.LoadingFragmentPagerActivity.invalidateFragments(LoadingFragmentPagerActivity.java:30)
at com.gh4a.activities.RepositoryActivity.onTreeSelected(RepositoryActivity.java:295)
at com.gh4a.fragment.ContentListFragment.onItemClick(ContentListFragment.java:104)
at com.gh4a.fragment.ContentListFragment.onItemClick(ContentListFragment.java:1)
at com.gh4a.fragment.ListDataBaseFragment.onListItemClick(ListDataBaseFragment.java:54)
at android.support.v4.app.ListFragment$2.onItemClick(ListFragment.java:58)
at android.widget.AdapterView.performItemClick(AdapterView.java:298)
at android.widget.AbsListView.performItemClick(AbsListView.java:1139)
at android.widget.AbsListView$PerformClick.run(AbsListView.java:3057)
at android.widget.AbsListView$4.run(AbsListView.java:3799)
at android.widget.AbsListView.onDetachedFromWindow(AbsListView.java:2923)
[...]
at android.view.ViewGroup.removeView(ViewGroup.java:3750)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:998)
at android.support.v4.app.FragmentManagerImpl.removeFragment(FragmentManager.java:1171)
[...]
at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:431)
at android.support.v4.app.FragmentStatePagerAdapter.finishUpdate(FragmentStatePagerAdapter.java:160)
at android.support.v4.view.ViewPager.dataSetChanged(ViewPager.java:703)
at android.support.v4.view.ViewPager$PagerObserver.onChanged(ViewPager.java:2060)
at android.database.DataSetObservable.notifyChanged(DataSetObservable.java:37)
at android.support.v4.view.PagerAdapter.notifyDataSetChanged(PagerAdapter.java:276)
at com.gh4a.LoadingFragmentPagerActivity.invalidateFragments(LoadingFragmentPagerActivity.java:30)
at com.gh4a.activities.RepositoryActivity.onBackPressed(RepositoryActivity.java:319)
[...]
Upgrade prettify to latest version.
Use screen estate better in diff view.

Shrink the huge padding to the left (and right), and make font sizes
somewhat consistent to prettified code output.

In case we need larger fonts at some point, this should probably happen
in the form of a default zoom level option.
Adjust foreground color of change headers in light theme.