checkpatch: don't try to get maintained status when --no-tree is given

Fixes the following warning:

  Use of uninitialized value $root in concatenation (.) or string at /path/to/checkpatch.pl line 764.

Link: http://lkml.kernel.org/r/1476719709-16668-1-git-send-email-jerome.forissier@linaro.org
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>