create_branch: drop unused "head" parameter

create_branch: drop unused "head" parameter

This function used to have the caller pass in the current
value of HEAD, in order to make sure we didn't clobber HEAD.
In 55c4a6730, that logic moved to validate_new_branchname(),
which just resolves HEAD itself. The parameter to
create_branch is now unused.

Since we have to update and re-wrap the docstring describing
the parameters anyway, let's take this opportunity to break
it out into a list, which makes it easier to find the
parameters.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>