Update GitGutter after Gwrite

GitGutter needs to be updated after `:Gwrite` is called.

I also added `<leader>gg` to toggle g(it)g(utter) visibility. All of the other `<leader>g_` shortcuts are related to fugitive, but I feel that it's best to just put the "git-related" shortcuts together. The `<leader>gg` mnemonic shouldn't conflict with any git commands other than grep, which is probably much less frequently used.