feat: add :Gdiff command for unified diff against git revision

Compares current buffer against any git revision (default HEAD), opens result
in vsplit with full diffs.nvim syntax highlighting.

spec/commands_spec.lua

@@ -0,0 +1,37 @@
+require('spec.helpers')
+
+describe('commands', function()
+  describe('setup', function()
+    it('registers Gdiff command', function()
+      require('diffs.commands').setup()
+      local commands = vim.api.nvim_get_commands({})
+      assert.is_not_nil(commands.Gdiff)
+    end)
+  end)
+
+  describe('unified diff generation', function()
+    local old_lines = { 'local M = {}', 'return M' }
+    local new_lines = { 'local M = {}', 'local x = 1', 'return M' }
+
+    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 = vim.diff(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 = vim.diff(content, content, {
+        result_type = 'unified',
+        ctxlen = 3,
+      })
+      assert.are.equal('', diff_output)
+    end)
+  end)
+end)