Make rscope usable throught tramp

Set default-directory to execute shell from it.
Amend the git root-tree find function for automatic cscope.out
generation.

Signed-off-by: Sylvain Chouleur <sylvain.chouleur@gmail.com>
[add git root-tree tramp code]
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>