fix: resolve relative includes from relative config paths

Co-authored-by: Byron <63622+Byron@users.noreply.github.com>

Author: Copilot

git/config.py

@@ -661,7 +661,8 @@ def read(self) -> None:  # type: ignore[override]
                             continue
                         # END ignore relative paths if we don't know the configuration file path
                         file_path = cast(PathLike, file_path)
-                        assert osp.isabs(file_path), "Need absolute paths to be sure our cycle checks will work"
+                        if not osp.isabs(file_path):
+                            file_path = osp.abspath(file_path)
                         include_path = osp.join(osp.dirname(file_path), include_path)
                     # END make include path absolute
                     include_path = osp.normpath(include_path)

test/test_config.py

@@ -246,6 +246,17 @@ def check_test_value(cr, value):
         with GitConfigParser(fpa, read_only=True) as cr:
             check_test_value(cr, tv)
 
+    @with_rw_directory
+    def test_relative_include_with_relative_config_path(self, rw_dir):
+        fpa = osp.join(rw_dir, "a")
+        fpb = osp.join(rw_dir, "b")
+        with GitConfigParser(fpb, read_only=False) as cw:
+            cw.set_value("b", "value", "b")
+        with GitConfigParser(fpa, read_only=False) as cw:
+            cw.set_value("include", "path", "b")
+        with GitConfigParser(osp.relpath(fpa), read_only=True) as cr:
+            assert cr.get_value("b", "value") == "b"
+
     @with_rw_directory
     def test_multiple_include_paths_with_same_key(self, rw_dir):
         """Test that multiple 'path' entries under [include] are all respected.