Merge pull request #1090 from MikeMcQuaid/brew-version-repository

brew.sh: fix brew --version outside HOMEBREW_REPOSITORY.

Library/Homebrew/brew.sh

-HOMEBREW_VERSION="$(git describe --tags --dirty 2>/dev/null)"
+HOMEBREW_VERSION="$(GIT_DIR="$HOMEBREW_REPOSITORY/.git" git describe --tags --dirty 2>/dev/null)"
 if [[ -z "$HOMEBREW_VERSION" ]]
 then
   HOMEBREW_VERSION=">1.0.0 (no git repository)"