[Windows] Manual update "git reset --hard" causes "volt get -u {repo}" always fail

[tyru]

In #49 branch:

1. cd ~/volt/repos/{repo}
2. git reset --hard HEAD~
3. volt get -u {repo}

{repo} : upgrade failed : worktree contains unstagged changes

After manual update of repository git reset --hard, volt get -u {repo} always fail.

[tyru]

This problem occurs the following environment:

• Windows Subsystem Linux
• ~/volt is symlink to /mnt/c/...

[tyru]

Maybe because it appears that file permission is changed:

$ git diff HEAD
diff --git a/README.md b/README.md
old mode 100644
new mode 100755

If I run git config core.filemode false in the repository, git diff or git status tell no changes,
but go-git might not support core.filemode ...?

[tyru]

go-git might not support core.filemode ...?

Bingo. src-d/go-git#378