build: propagate $DIFF to scripts

git-merge-one-file expects to run "-u" capable "diff", but using
$DIFF is not the right way to do so.

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

Makefile

@@ -1494,6 +1494,7 @@ SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))
 PERL_PATH_SQ = $(subst ','\'',$(PERL_PATH))
 PYTHON_PATH_SQ = $(subst ','\'',$(PYTHON_PATH))
 TCLTK_PATH_SQ = $(subst ','\'',$(TCLTK_PATH))
+DIFF_SQ = $(subst ','\'',$(DIFF))
 
 LIBS = $(GITLIBS) $(EXTLIBS)
 
@@ -1582,6 +1583,7 @@ define cmd_munge_script
 $(RM) $@ $@+ && \
 sed -e '1s|#!.*/sh|#!$(SHELL_PATH_SQ)|' \
     -e 's|@SHELL_PATH@|$(SHELL_PATH_SQ)|' \
+    -e 's|@@DIFF@@|$(DIFF_SQ)|' \
     -e 's/@@GIT_VERSION@@/$(GIT_VERSION)/g' \
     -e 's/@@NO_CURL@@/$(NO_CURL)/g' \
     -e $(BROKEN_PATH_FIX) \

git-merge-one-file.sh

@@ -107,7 +107,7 @@ case "${1:-.}${2:-.}${3:-.}" in
 		# remove lines that are unique to ours.
 		orig=`git-unpack-file $2`
 		sz0=`wc -c <"$orig"`
-		$DIFF -u -La/$orig -Lb/$orig $orig $src2 | git apply --no-add
+		@@DIFF@@ -u -La/$orig -Lb/$orig $orig $src2 | git apply --no-add
 		sz1=`wc -c <"$orig"`
 
 		# If we do not have enough common material, it is not