--diff outputs absolute paths

[akaihola]

In #4, we added the --diff flag to mirror black --diff.

However, I now notice that while Black outputs paths relative to the current working directory, Darker now outputs absolute paths.

This might lead to incompatibilities with some tools which expect relative paths.

FYI @Carreau

$ black --check --diff --quiet src/darker/tests/test_black_diff.py
--- src/darker/tests/test_black_diff.py	2020-08-04 16:51:03.924124 +0000
+++ src/darker/tests/test_black_diff.py	2020-08-04 16:56:28.621010 +0000
@@ -1,10 +1,10 @@
 from pathlib import Path
 
 import pytest
 
-from  darker.black_diff import BlackArgs, read_black_config, run_black
+from darker.black_diff import BlackArgs, read_black_config, run_black
 
 
 @pytest.mark.parametrize(
     "config_path, config_lines, expect",
     [

$ darker --diff --quiet src/darker/tests/test_black_diff.py     
--- /home/akaihola/prg/darker/src/darker/tests/test_black_diff.py
+++ /home/akaihola/prg/darker/src/darker/tests/test_black_diff.py
@@ -2,7 +2,7 @@

 
 import pytest
 
-from  darker.black_diff import BlackArgs, read_black_config, run_black
+from darker.black_diff import BlackArgs, read_black_config, run_black
 
 
 @pytest.mark.parametrize(