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>

.gitattributes

diff --git a/.gitattributes b/.gitattributes
index 59cd41d..118d56c 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -3,3 +3,4 @@
 git-gui.sh  encoding=UTF-8
 /po/*.po    encoding=UTF-8
 /GIT-VERSION-GEN eol=lf
+Makefile    whitespace=!indent,trail,space