brew-build-bottle-pr: improvements
brew-build-bottle-pr: improvements

* Reorganize the code to make changes to formula *after* we switch to a
new branch so that we don't leave uncommitted changes if something fails
* Delete local `bottle-<formula>` branch if it exists (requires --force)
* Delete remote bottle-<formula> branch in your-fork remoteif it exists
(requires --force)

Closes Linuxbrew/homebrew-developer#28

Signed-off-by: Maxim Belkin <032baa6ccf1d4eaa4536fc4f9b83b9a1f593449a@gmail.com>
Linuxbrew/homebrew-developer