re-generate ChangeLog only if it's newer than the git index

otherwise, we'd re-generate it during 'install' as well.

note that this does not work with new-style git worktrees, where .git is
only a file - but there the log generation itself already doesn't work
anyway.

Makefile.am

@@ -47,7 +47,10 @@ LOG_PL = \
         print $$log."\n"; \
     }
 
-$(srcdir)/ChangeLog: log
+$(srcdir)/.git/index:
+$(srcdir)/ChangeLog: $(srcdir)/.git/index
+	$(MAKE) log
+
 log:
 	@test -z "$(srcdir)" || cd $(srcdir) && \
 	 ( ! test -d .git || \