Poor support for the framework submodule in code_style.py

[gilles-peskine-arm]

code_style.py doesn't support the framework very well:

• It doesn't work from a Git hook. Fixed in Allow code_style.py to work from a git hook mbedtls#9233.
• code_style.py --since REVISION passes REVISION unchanged when it runs into the submodule, instead of using the submodule sha corresponding to REVISION in the main repository.