Blank commit created if the command results in no changes

[motiz88]

Originally reported by @Stupidism in #5 (comment):

When there's nothing to commit after cmd executed(original diff is fixed back to unchanged), the commit is blank now. It should abort commit process by exit 1

[Stupidism]

How to reproduce:

1. make an auto-fixable change (e.g. remove a trail-coma or a ;)
2. git add . && git commit
3. git reset --mixed back to last commit, you will find there's nothing changed in the commit

[motiz88]

I see. This functionality would ideally need to interop with git commit --allow-empty somehow. I wonder if we can detect this flag in a hook.