.gitignore: Add *~, *.orig, *.rej, move to root directory

Move the .gitignore out of the Marlin subdirectory so it applies to
the whole tree, and add some missing patterns.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

.gitignore

+*.o
+applet/
+*~
+*.orig
+*.rej

Marlin/.gitignore

-*.o
-applet/