ansible-test - change diff mathching regexp so prefixes are optional

The `a/` and `b/` prefixes can be disabled in the `git diff` output by setting
`diff.noprefix` to `true`. The output is still a valid diff, but `ansible-test` would
raise an exception since without the prefixes, it thought the diff line was invalid.

changelogs/fragments/ansible-test-diff-prefix-optional.yml

@@ -0,0 +1,4 @@
+bugfixes:
+  - >-
+    ansible-test - make the ``a/`` and ``b/`` prefixes an optional match
+    since these can be turned off with the ``diff.noprefix`` setting in ``git``

test/lib/ansible_test/_internal/diff.py

@@ -180,7 +180,7 @@ class DiffParser:
         """Process a diff start line."""
         self.complete_file()
 
-        match = re.search(r'^diff --git "?a/(?P<old_path>.*)"? "?b/(?P<new_path>.*)"?$', self.line)
+        match = re.search(r'^diff --git "?(?:a/)?(?P<old_path>.*)"? "?(?:b/)?(?P<new_path>.*)"?$', self.line)
 
         if not match:
             raise Exception('Unexpected diff start line.')