Add .depend directories to .gitignore

The makefile snippets that would land in these directories are already
being ignored.  Ignore the directories instead so they don’t show up
in ‘git clean -n’ output.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.gitignore

@@ -178,7 +178,7 @@
 *.exe
 *.[aos]
 *.py[co]
-*.o.d
+.depend/
 *+
 /config.mak
 /autom4te.cache