.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

@@ -0,0 +1,5 @@
+*.o
+applet/
+*~
+*.orig
+*.rej

Marlin/.gitignore

@@ -1,2 +0,0 @@
-*.o
-applet/