Makefile: don't use "FORCE" for tags targets

Remove the "FORCE" dependency from the "tags", "TAGS" and "cscope"
targets, instead make them depend on whether or not the relevant
source files have changed.

For the cscope target we need to change it to depend on the actual
generated file while we generate while we're at it, as the next commit
will discuss we always generate a cscope.out file.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2727,20 +2727,24 @@ FIND_SOURCE_FILES = ( \
 		| sed -e 's|^\./||' \
 	)
 
-$(ETAGS_TARGET): FORCE
+FOUND_SOURCE_FILES = $(shell $(FIND_SOURCE_FILES))
+
+$(ETAGS_TARGET): $(FOUND_SOURCE_FILES)
 	$(QUIET_GEN)$(RM) "$(ETAGS_TARGET)+" && \
-	$(FIND_SOURCE_FILES) | xargs etags -a -o "$(ETAGS_TARGET)+" && \
+	echo $(FOUND_SOURCE_FILES) | xargs etags -a -o "$(ETAGS_TARGET)+" && \
 	mv "$(ETAGS_TARGET)+" "$(ETAGS_TARGET)"
 
-tags: FORCE
+tags: $(FOUND_SOURCE_FILES)
 	$(QUIET_GEN)$(RM) tags+ && \
-	$(FIND_SOURCE_FILES) | xargs ctags -a -o tags+ && \
+	echo $(FOUND_SOURCE_FILES) | xargs ctags -a -o tags+ && \
 	mv tags+ tags
 
-.PHONY: cscope
-cscope:
+cscope.out: $(FOUND_SOURCE_FILES)
 	$(QUIET_GEN)$(RM) cscope* && \
-	$(FIND_SOURCE_FILES) | xargs cscope -b
+	echo $(FOUND_SOURCE_FILES) | xargs cscope -b
+
+.PHONY: cscope
+cscope: cscope.out
 
 ### Detect prefix changes
 TRACK_PREFIX = $(bindir_SQ):$(gitexecdir_SQ):$(template_dir_SQ):$(prefix_SQ):\
@@ -2921,7 +2925,7 @@ check: config-list.h command-list.h
 		exit 1; \
 	fi
 
-FOUND_C_SOURCES = $(filter %.c,$(shell $(FIND_SOURCE_FILES)))
+FOUND_C_SOURCES = $(filter %.c,$(FOUND_SOURCE_FILES))
 COCCI_SOURCES = $(filter-out $(THIRD_PARTY_SOURCES),$(FOUND_C_SOURCES))
 
 %.cocci.patch: %.cocci $(COCCI_SOURCES)