Allow creation of new branch with `fold`

[narnaud]

To create a new branch from a commit, one can use: loom branch <branch_name> -t <sha1>
But the --target flag is not really explicit, and once used to fold it makes sense to use this command.

Add a --create flag to create a new branch if the branch name does not exist:
loom fold -c <sha> <branch_name>which will be equivalent to:loom fold <branch_name> -t `