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/git_spec.lua

@@ -0,0 +1,68 @@
+require('spec.helpers')
+local git = require('diffs.git')
+
+describe('git', function()
+  describe('get_git_dir', function()
+    it('returns git dir for current repo', function()
+      local cwd = vim.fn.getcwd()
+      local git_dir = git.get_git_dir(cwd .. '/lua/diffs/init.lua')
+      assert.is_not_nil(git_dir)
+      assert.is_true(vim.fn.isdirectory(git_dir) == 1)
+    end)
+
+    it('returns nil for non-git directory', function()
+      local git_dir = git.get_git_dir('/tmp')
+      assert.is_nil(git_dir)
+    end)
+  end)
+
+  describe('get_repo_root', function()
+    it('returns repo root for current repo', function()
+      local cwd = vim.fn.getcwd()
+      local root = git.get_repo_root(cwd .. '/lua/diffs/init.lua')
+      assert.is_not_nil(root)
+      assert.are.equal(cwd, root)
+    end)
+
+    it('returns nil for non-git directory', function()
+      local root = git.get_repo_root('/tmp')
+      assert.is_nil(root)
+    end)
+  end)
+
+  describe('get_file_content', function()
+    it('returns file content at HEAD', function()
+      local cwd = vim.fn.getcwd()
+      local content, err = git.get_file_content('HEAD', cwd .. '/lua/diffs/init.lua')
+      assert.is_nil(err)
+      assert.is_not_nil(content)
+      assert.is_true(#content > 0)
+    end)
+
+    it('returns error for non-existent file', function()
+      local cwd = vim.fn.getcwd()
+      local content, err = git.get_file_content('HEAD', cwd .. '/does_not_exist.lua')
+      assert.is_nil(content)
+      assert.is_not_nil(err)
+    end)
+
+    it('returns error for non-git directory', function()
+      local content, err = git.get_file_content('HEAD', '/tmp/some_file.txt')
+      assert.is_nil(content)
+      assert.is_not_nil(err)
+    end)
+  end)
+
+  describe('get_relative_path', function()
+    it('returns relative path within repo', function()
+      local cwd = vim.fn.getcwd()
+      local rel = git.get_relative_path(cwd .. '/lua/diffs/init.lua')
+      assert.are.equal('lua/diffs/init.lua', rel)
+    end)
+
+    it('returns nil for non-git directory', function()
+      local rel = git.get_relative_path('/tmp/some_file.txt')
+      assert.is_nil(rel)
+    end)
+  end)
+end)