Makefile: Hide away auto-generated dependency file as .depends.

Instead of the old name of Makefile.dep. The idea being that the
user really doesn't need to see this by default, (and if debugging
the Makefile, the rules will make the name obvious).

.gitignore

@@ -1,4 +1,4 @@
-Makefile.dep
+.depends
 notmuch
 notmuch.1.gz
 

Makefile

@@ -37,10 +37,10 @@ all: $(PROGS)
 notmuch: $(MAIN) $(LIBRARY)
 	$(CC) $(NOTMUCH_LDFLAGS) $^ -o $@
 
-Makefile.dep: *.c lib/*.c lib/*.cc
+.depends: *.c lib/*.c lib/*.cc
 	$(CXX) -M $(CPPFLAGS) $(NOTMUCH_DEPENDS_FLAGS) \
 	$(NOTMUCH_CXX_DEPENDS_FLAGS) $^ > $@
--include Makefile.dep
+-include .depends
 
 notmuch.1.gz:
 	gzip --stdout notmuch.1 > notmuch.1.gz
@@ -52,4 +52,4 @@ install: all notmuch.1.gz
 		$(DESTDIR)/etc/bash_completion.d/notmuch
 
 clean:
-	rm -f $(PROGS) lib/*.o *.o Makefile.dep
+	rm -f $(PROGS) lib/*.o *.o .depends