feat: add :Gdiff, :Gvdiff, :Ghdiff commands for unified diff view

Compares current buffer against any git revision (default HEAD), opens result
with full diffs.nvim syntax highlighting. Follows fugitive convention:
:Gdiff/:Gvdiff open vertical split, :Ghdiff opens horizontal split.

spec/commands_spec.lua

@@ -0,0 +1,40 @@
+require('spec.helpers')
+
+describe('commands', function()
+  describe('setup', function()
+    it('registers Gdiff, Gvdiff, and Ghdiff commands', function()
+      require('diffs.commands').setup()
+      local commands = vim.api.nvim_get_commands({})
+      assert.is_not_nil(commands.Gdiff)
+      assert.is_not_nil(commands.Gvdiff)
+      assert.is_not_nil(commands.Ghdiff)
+    end)
+  end)
+
+  describe('unified diff generation', function()
+    local old_lines = { 'local M = {}', 'return M' }
+    local new_lines = { 'local M = {}', 'local x = 1', 'return M' }
+    local diff_fn = vim.text and vim.text.diff or vim.diff
+
+    it('generates valid unified diff', function()
+      local old_content = table.concat(old_lines, '\n')
+      local new_content = table.concat(new_lines, '\n')
+      local diff_output = diff_fn(old_content, new_content, {
+        result_type = 'unified',
+        ctxlen = 3,
+      })
+      assert.is_not_nil(diff_output)
+      assert.is_true(diff_output:find('@@ ') ~= nil)
+      assert.is_true(diff_output:find('+local x = 1') ~= nil)
+    end)
+
+    it('returns empty for identical content', function()
+      local content = table.concat(old_lines, '\n')
+      local diff_output = diff_fn(content, content, {
+        result_type = 'unified',
+        ctxlen = 3,
+      })
+      assert.are.equal('', diff_output)
+    end)
+  end)
+end)