Fix warning 'function': incompatible types - from 'git_cvar_value *' to 'int *' (C4133) on VS - #4926

This error message sounds like what MSVC prints when it doesn't know about a type and thus it defaults to int (because of some ancient C rule).

I'm okay with taking this to squash the warning, but at a glance, I don't know why git_repository__cvar takes an int * not a git_cvar_value * and that seems like a reasonable long-term change.

Since this is a config cache, sometimes the values that we're caching are actually ints and not things that map to enums, so actually using git_cvar_value here is not actually appropriate. Merging this, and I'm leaving the rest as-is.