Don't assume the default branch is named `master`

[nhasselmeyer]

A lot of commands in geordi still assume that the default git branch is named master, which is slightly outdated.

From grepping through the project, the affected commands are at least

• geordi security_update
• geordi branch -m (the long form of the argument should also be aliased as --from-main)
• geordi deploy

As this makes the mentioned commands less useful or unusable in newer projects, these commands should check git config init.defaultBranch instead of using master.