git-gui: Give a better error message on an empty branch name.

git-gui: Give a better error message on an empty branch name.

New branches must have a name.  An empty one is not a valid ref, but the
generic message "We do not like '' as a branch name." is just too vague
or difficult to read.  So detect the missing name early and tell the
user it must be entered.

Signed-off-by: Shawn O. Pearce <redacted>