Makefile(s): do not enforce "all indents must be done with tab"

Our top-level Makefile follows our generic whitespace rule established by the top-level .gitattributes file that does not enforce indent-with-non-tab rule by default, but git-gui is set up to enforce indent-with-non-tab by default. With the upcoming change to GNU make, we no longer can reject (and worse, "fix") a patch that adds whitespace indented lines to the Makefile, so loosen the rule there for git-gui/Makefile, too. Signed-off-by: Junio C Hamano <gitster@pobox.com>