Fix "ambiguous argument 'origin/master'" error.

When installing Homebrew, I get the following error:

```shell
$ ruby -e "$(curl -fsSL https://raw.github.com/Homebrew/homebrew/go/install)"
==> This script will install:
/usr/local/bin/brew
/usr/local/Library/...
/usr/local/share/man/man1/brew.1

Press RETURN to continue or any other key to abort
==> /usr/bin/sudo /bin/chmod g+rwx /Library/Caches/Homebrew
==> Downloading and installing Homebrew...
remote: Counting objects: 177932, done.
remote: Compressing objects: 100% (49344/49344), done.
remote: Total 177932 (delta 127453), reused 177895 (delta 127426)
Receiving objects: 100% (177932/177932), 35.28 MiB | 135.00 KiB/s, done.
Resolving deltas: 100% (127453/127453), done.
From https://github.com/Homebrew/homebrew
 * [new branch]      master     -> origin/master
 x [deleted]         (none)     -> origin/master
fatal: ambiguous argument 'origin/master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Failed during: git reset --hard origin/master
```

This proposed change fixes the problem.