Add support for a mode where positionals are not positional

[felixSchl]

This is for convenience to the user:

usage: git branch [options] [-r | -a] [--merged | --no-merged]
   or: git branch [options] [-l] [-f] <branchname> [<start-point>]
   or: git branch [options] [-r] (-d | -D) <branchname>...
   or: git branch [options] (-m | -M) [<oldbranch>] <newbranch>

e.g. git branch -m, this all works:

git branch -m foo bar
git branch foo -m bar
git branch foo bar -m

This makes it easy to add another flag at then end should the user forget one.

[felixSchl]

Work is tracked on feature/free-positionals and almost ready

[felixSchl]

Landed via 773941c