Check early that a new branch is new and valid

Check early that a new branch is new and valid

If you fail to update refs to change branches in checkout, your index
and working tree are left already updated. We don't have an easy way
to undo this, but at least we can check things that would make the
creation of a new branch fail. These checks were in the shell version,
and were lost in the C conversion.

The messages are from the shell version, and should probably be made nicer.

[jc: added test to t7201]

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>