pytest_plugin(fix[gitconfig]): Allow file:// protocol for submodule tests

why: git submodule operations spawn child processes that don't inherit
local repo config, causing "transport 'file' not allowed" errors in
strict build environments (#509)
what:
- Add [protocol "file"] allow = always to gitconfig fixture

Author: tony

src/libvcs/pytest_plugin.py

@@ -165,6 +165,8 @@ def gitconfig(
     name = {vcs_name}
   [color]
     diff = auto
+  [protocol "file"]
+    allow = always
     """,
         ),
         encoding="utf-8",