Allow creating branches without committing in fast-import.

Allow creating branches without committing in fast-import.

Some importers may want to create a branch long before they actually
commit to it, or in some cases they may never commit to the branch
but they still need the ref to be created in the repository after
the import is complete.

This extends the 'reset ' command to automatically create a new
branch if the supplied reference isn't already known as a branch.

While I'm at it I also modified the syntax of the reset command
to terminate with an empty line, like commit and tag operate.
This just makes the command set more consistent.

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