refactor(config): nest integration toggles under integrations namespace (#174)

## Problem

Integration keys (`fugitive`, `neogit`, `gitsigns`, `committia`,
`telescope`)
live at the top level of `vim.g.diffs`, cluttering the config namespace.

## Solution

Move them under `vim.g.diffs.integrations.*`. Old top-level keys still
work but
emit `vim.deprecate` targeting v0.3.2. `compute_filetypes` and
`plugin/diffs.lua`
fall back to legacy keys for pre-`init()` callers.

Also includes `83c17ac` which fixes an invalid hex hash in the combined
diff test
fixture.

spec/neogit_integration_spec.lua

@@ -1,6 +1,6 @@
 require('spec.helpers')
 
-vim.g.diffs = { neogit = true }
+vim.g.diffs = { integrations = { neogit = true } }
 
 local diffs = require('diffs')
 local parser = require('diffs.parser')