Add unified diff support for luatest.assert_equals

This patch adds unified diff output for `t.assert_equals()` failures when
the test suite is run with the `--diff` option, using a vendored Lua
implementation of google/diff-match-patch (`luatest/vendor/diff_match_patch.lua`
taken from [^1]).

Closes #412

[^1]: https://github.com/google/diff-match-patch/blob/master/lua/diff_match_patch.lua

Author: mandesero

.luacheckrc

@@ -1,4 +1,4 @@
 include_files = {"**/*.lua", "*.rockspec", "*.luacheckrc"}
-exclude_files = {"build.luarocks/", "lua_modules/", "tmp/", ".luarocks/", ".rocks/"}
+exclude_files = {"build.luarocks/", "lua_modules/", "tmp/", ".luarocks/", ".rocks/", "luatest/vendor/"}
 
 max_line_length = 120

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- Added support for unified diff output in `t.assert_equals()` failure messages
+  when expected and actual values are YAML-serializable (gh-412).
+
 ## 1.3.0
 
 - Fixed a bug when `assert_covers` treats arrays as maps (gh-405).

luatest/assertions.lua

@@ -6,6 +6,7 @@
 local math = require('math')
 
 local comparator = require('luatest.comparator')
+local diff = require('luatest.diff')
 local mismatch_formatter = require('luatest.mismatch_formatter')
 local pp = require('luatest.pp')
 local log = require('luatest.log')
@@ -20,12 +21,17 @@ local prettystr_pairs = pp.tostring_pair
 local M = {}
 
 local xfail = false
+local diff_enabled = true
 
 local box_error_type = ffi.typeof(box.error.new(box.error.UNKNOWN))
 
 -- private exported functions (for testing)
 M.private = {}
 
+function M.private.set_diff_enabled(value)
+    diff_enabled = value and true or false
+end
+
 function M.private.is_xfail()
     local xfail_status = xfail
     xfail = false
@@ -83,6 +89,14 @@ local function error_msg_equality(actual, expected, deep_analysis)
         if success then
             result = table.concat({result, mismatchResult}, '\n')
         end
+
+        if diff_enabled then
+            local diff_result = diff.build_unified_diff(expected, actual)
+            if diff_result then
+                result = table.concat({result, 'diff:', diff_result}, '\n')
+            end
+        end
+
         return result
     end
     return string.format("expected: %s, actual: %s",