Some git scripts cleanup.

Mainly about the default branch, fetching and rebasing on it.

- A nicer default-branch alias (without a shell).
- Address if running git-master on the default branch.
- Make git-master a script since it became too complex.
- Address if running git-remaster on the default branch.
- Make git-remaster a script sine it became too complex.
- Remove git-rerebase (I should just use git pull --rebase).
- Change git-ff to use a non-hardcoded default branch.