Use git worktrees for foreman-packaging

Previously it was assumed to have something like ~/dev/foreman-packaging
but with git worktrees you can create multiple working directories. I
have switched over to:

    $ tree -L 1 ~/dev/foreman-packaging
    /home/ekohl/dev/foreman-packaging
    ├── deb-3.1
    ├── deb-3.2
    ├── deb-3.3
    ├── deb-3.4
    ├── deb-3.5
    ├── deb-3.6
    ├── deb-develop
    ├── master
    ├── rpm-3.1
    ├── rpm-3.2
    ├── rpm-3.3
    ├── rpm-3.5
    ├── rpm-3.6
    └── rpm-develop

This is all backed by a single .git directory, which makes it much
cheaper.

bump_packaging

@@ -12,7 +12,16 @@ fi
 REMOTE_BRANCH="${FLAVOR}/${FOREMAN_VERSION}"
 FULL_REMOTE_BRANCH="$PACKAGING_GIT_REMOTE/$REMOTE_BRANCH"
 
-cd "$PACKAGING_DIR"
+RELEASE_PACKAGING_DIR="${PACKAGING_DIR}/${FLAVOR}-$VERSION"
+
+if [[ ! -d "$RELEASE_PACKAGING_DIR" ]] ; then
+	(
+		cd "${PACKAGING_DIR}/master"
+		git worktree add "$RELEASE_PACKAGING_DIR" "$PACKAGING_GIT_REMOTE/$REMOTE_BRANCH"
+	)
+fi
+
+cd "$RELEASE_PACKAGING_DIR"
 
 git fetch $PACKAGING_GIT_REMOTE
 git checkout -b "${FLAVOR}/release-$PROJECT-$FULLVERSION" "${FULL_REMOTE_BRANCH}"