Revert gitsigns keymaps but fix vimdiff and fugitive conflict

Originally, the keymaps for jumping to next and previous git hunks were
]c and [c. This was changed in #323 (83b65a1) because they overwrote the
built-in vimdiff keymaps.

However, the more traditional solution is to have ]c and [c *extend* the
built-in keymap. This is what fugitive and gitgutter have been doing for
years.

Gitsigns doesn't do this by itself, but it has a recommended keymap
configuration on which the present patch is based:

	https://github.com/lewis6991/gitsigns.nvim#keymaps

The only thing I've added is to have the keymaps work in visual mode as
well, which is the same behavior as the built in vimdiff keymaps.

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