scripts/kernel_bump: Use the git index to find the needed config files

scripts/kernel_bump: Use the git index to find the needed config files

The current solution using `find` introduces a racecondition, where `find`
and `git mv` get in each others way. While this could be fixed with
more-utils sponge command (or even sort -u) to buffer the output of
find.

However, a much better approach, is to query the git index directly,
which will not change, and is far more accurate.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>