Rename 's_diffmap' to 's_diff_map'

Keep naming consistency.

https://bugzilla.gnome.org/show_bug.cgi?id=720726
Don't open WebKit inspector at the start

As we are using WebView in both history and commit activties, it is
annoying to see two WebKit inspector windows open when Gitg starts.

https://bugzilla.gnome.org/show_bug.cgi?id=720726
Document why we call 'update_diff()' here

https://bugzilla.gnome.org/show_bug.cgi?id=720726