Fixed #75.

* util/git-checkout.sh: Newly added.
* binutils/checkout.sh: Modified to delegate most of the work to
  `util/git-checkout.sh`.
Refactored.

* util/git-checkout.sh: Refactored.
* binutils/checkout.sh: Likewise.
Merge pull request #97 from Cryolite/fix/75

Fixed #75.