Makefile: Copy only files to /usr/bin

The 'cp -r' command copies the git-log directory to /usr/bin as well,
which is redundant. With 'find' we can copy the files recursively to
the destination.

Also, added a -R option to chmod to set permissions recursively.

Signed-off-by: Thameera Senanayaka <thameera123@gmail.com>