Remove branch creation command from fast-import.

Remove branch creation command from fast-import.

Jon Smirl was finding it difficult to alter cvs2svn to generate
branch commands prior to the first commit of the same branch.
This change moves the 'from' command to be an optional parameter of
the 'commit' command, thereby allowing a new branch to be defined
at the moment it gets used to create the first commit on that branch.

This change makes it impossible to create a branch with no commits
on it as at least one commit is needed to register the branch.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>