Improve support for releasing additional files

[fredemmott]

Current process is:

• locally creating a branch just before the commit that does not apply
• run with --create-fixup-patch --source-repo-dir=PATH_TO_REPO_WHERE_YOU_MADE_THE_BRANCH --source-branch=BRANCH_YOU_JUST_CREATED
• apply it to the git dir with patch
• review the changes to make sure there's nothing fbonly
• commit the result and push directly to github

Ideas:

• add support for specifying an initial revision so the branch doesn't need to be created
• add an option to automatically apply the patch, and dump people into git add --patch for review

Optional full automation will not be accepted.

[fredemmott]

Could also be useful for --create-fixup-patch to make something suitable for git-am, including the fbshipit-sync-id header