Fix an issue with creatin a diff, fixes #1757

jedi/api/refactoring/__init__.py

@@ -42,11 +42,17 @@ class ChangedFile:
         if self._from_path is None:
             from_p = ''
         else:
-            from_p = self._from_path.relative_to(project_path)
+            try:
+                from_p = self._from_path.relative_to(project_path)
+            except ValueError:  # Happens it the path is not on th project_path
+                from_p = self._from_path
         if self._to_path is None:
             to_p = ''
         else:
-            to_p = self._to_path.relative_to(project_path)
+            try:
+                to_p = self._to_path.relative_to(project_path)
+            except ValueError:
+                to_p = self._to_path
         diff = difflib.unified_diff(
             old_lines, new_lines,
             fromfile=str(from_p),

test/test_api/test_refactoring.py

@@ -70,3 +70,19 @@ def test_diff_without_ending_newline(Script):
         -a
         +c
         ''')
+
+
+def test_diff_path_outside_of_project(Script):
+    script = Script(
+        code='foo = 1',
+        path='/unknown_dir/file.py',
+        project=jedi.get_default_project()
+    )
+    diff = script.rename(line=1, column=0, new_name='bar').get_diff()
+    assert diff == dedent('''\
+        --- /unknown_dir/file.py
+        +++ /unknown_dir/file.py
+        @@ -1 +1 @@
+        -foo = 1
+        +bar = 1
+        ''')