Improved C89 compliance: Introduce new scope before declaring variables

Further work on getting Git::Raw to build with older compilers. This change introduces scope directly after ```CODE:``` blocks. Without this, older compilers complain about declaring variables in the middle of a function as the XS translator may generate some code (during .xs to .c generation) before the variables that we declare. 

There is still some re-ordering of variable declarations to be done which I'll submit as a separate pull request.