This PR tries to implement the core.filemode and fix the issue 771, which addresses compatibility issues of go-git on Windows.

C git implements core.filemode in two ways:

1. Detect via flipping the executable bit and set at the beginning of init/clone
2. Compute the effective mode value from index via st_mode_from_ce depending on the value of trust_executable_bit, which derives from core.filemode

I've tried to align the implementation with how C git handles core.filemode to ensure consistency:

1. PlainInit/PlainClone will detect and set the value of core.filemode in .git/config
2. core.filemode(trustExecutableBit) only takes effect during hash comparison. When an index entry is an
   executable and trustExecutableBit is false, a mode value of 0644 will be used for hashing purposes—while the actual mode stored in the index remains unchanged.