Fix and clean top .gitignore

Removed vmlinux* rule because it matches too many useful files, replacing
it with rules matching filetype by filename (e.g. *.gz). Also unignored
.mailmap from the top directory. Added a comment telling the user how to
check for tracked files being ignored.

Signed-off-by: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>