This PR tries to implement the core.filemode related to the Windows issue 771. In this PR, the core.filemode option in config will be honored and only take effect during hash comparison. When an index entry is an
executable and core.filemode is false, a mode value of 0644 will be used for hashing purposes—while the actual mode value stored in the index remains unchanged.

It is true by default and hence the actual mode value will be used in hash.

AFAIK, C git will automatically set this option before init/clone, and git init results in a value of false by default on Windows. Unlike C git, there is no such auto detection in go-git by now. For windows users, if they want to use go-git to init or clone a repo on disk, they should explicitly set this config option and flush the config to disk.

    repository, err := git.PlainInit(repositoryPath, false)
	if err != nil {
		// error handling
	}
	cfg, err := repository.Config()
	if err != nil {
		// error handling
	}
	cfg.Core.FileMode = false
	err = repository.SetConfig(cfg)
	if err != nil {
		// error handling
	}