Fix GIT_MERGE_OPTS_INIT on MSVC.

On Visual Studio 2013, attempting to use `GIT_MERGE_OPTS_INIT` causes the following error:

```
error C2440: 'initializing' : cannot convert from 'int' to 'git_merge_flags_t'
Conversion to enumeration type requires an explicit cast (static_cast, C-style cast or function-style cast)
```

This pull request just adds an explicit cast to the default merge flags, which fixes this error. It doesn't affect compilation on GCC 4.7.2, nor should it on any standards-compliant compiler.