rebase -i: check labels and refs when parsing todo list

rebase -i: check labels and refs when parsing todo list

Check that the argument to the "label" and "update-ref" commands is a
valid refname when the todo list is parsed rather than waiting until the
command is executed. This means that the user can deal with any errors
at the beginning of the rebase rather than having it stop halfway
through due to a typo in a label name. The "update-ref" command is
changed to reject single level refs as it is all to easy to type
"update-ref branch" which is incorrect rather than "update-ref
refs/heads/branch"

Note that it is not straight forward to check the arguments to "reset"
and "merge" commands as they may be any revision, not just a refname and
we do not have an equivalent of check_refname_format() for revisions.

Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>