[USABILITY] git-reset removes directories

Ok I have been bitten by this issue twice. :(

`git reset --hard` will reset modifications in the working directory (of course). But if I remove a file, add a directory with the same name, `git reset --hard` will erase that whole directory. Specifically the following steps: ``` touch file git add file git commit -m "add file" # now the HEAD is constructed rm file mkdir file touch file/a # now here is the issue git reset --hard # this will remove the directory and all its files ```

I guess this has to do with Git not tracking "changing a file to a directory", especially because one is a blob and another is a tree.

In case of `git reset --hard`, people would expect reverting modifications, bringing up removed files, but not removing a whole directory with the same name. Especially if the directory is already populated. Then the whole thing is gone. I think it would better if Git could output a warning and abort, like in case of checkout ("Updating the following ... would lose ...").

But honestly I usually take `git reset --hard` seriously and will double check before invocation. What actually bit me was `git stash`. It will call `git reset --hard -q` after saving the working directory. But in the case I just described, since the new directory is not tracked, it will not be saved and will be erased. `git stash` usually gives me the mental model of "saving stuff in working directory", but not like in this case where it also deletes stuff in working directory.