Makefile: ask "ls-files" to list source files if available

The [ce]tags and cscope targets used to run "find" looking for any paths
that match '*.[chS]' to feed the list of source files to downstream xargs.

Use "git ls-files" if it is already available to us, and otherwise use a
tighter "find" expression that does not list directories and does not go
into our .git directory.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2115,17 +2115,21 @@ po/git.pot: $(LOCALIZED_C)
 
 pot: po/git.pot
 
+FIND_SOURCE_FILES = ( git ls-files '*.[hcS]' 2>/dev/null || \
+			$(FIND) . \( -name .git -type d -prune \) \
+				-o \( -name '*.[hcS]' -type f -print \) )
+
 $(ETAGS_TARGET): FORCE
 	$(RM) $(ETAGS_TARGET)
-	$(FIND) . -name '*.[hcS]' -print | xargs etags -a -o $(ETAGS_TARGET)
+	$(FIND_SOURCE_FILES) | xargs etags -a -o $(ETAGS_TARGET)
 
 tags: FORCE
 	$(RM) tags
-	$(FIND) . -name '*.[hcS]' -print | xargs ctags -a
+	$(FIND_SOURCE_FILES) | xargs ctags -a
 
 cscope:
 	$(RM) cscope*
-	$(FIND) . -name '*.[hcS]' -print | xargs cscope -b
+	$(FIND_SOURCE_FILES) | xargs cscope -b
 
 ### Detect prefix changes
 TRACK_CFLAGS = $(CC):$(subst ','\'',$(ALL_CFLAGS)):\