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. [j6t: cherry-picked from 227b8fd90240] Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Johannes Sixt <j6t@kdbg.org>