utils/check_patch.py: Speed and memory usage improvements

Turns out we had a bug in the code that verified whether
a file was tracked by git. With the bug fixed, a lot less
files will be scanned. Moreover, expand the list of
blacklisted extensions and paths, making it faster to
check the tree, even if you have logs in it.

Also, instead of creating multiple file checker
objects, that will be stored in memory, refactor
the existing one to process several files and
use it. This greatly saves memory.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>