test_cmp: do not use "diff -u" on platforms that lack one

By default the testsuite calls 'diff -u' whenever a file comparison is
called for.  Unfortunately that throws a "diff: unknown option '-u'"
error for most non-GNU diffs.

This patch sets GIT_TEST_CMP to 'cmp' on all the architectures where
that happens.  The previous version of this patch forgot to export
GIT_TEST_CMP from t/Makefile, which is why 'make test' continued to
fail most tests on most architectures - test-lib.sh was falling back
on its default of `diff -u' for GIT_TEST_CMP.  This version of this
patch shows a vast improvement in testsuite results where either GNU
diff is in the path at configure time, or where Makefile knows that
GIT_TEST_CMP=cmp is required.

Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -815,18 +815,21 @@ ifeq ($(uname_S),SunOS)
 		NO_STRLCPY = YesPlease
 		NO_C99_FORMAT = YesPlease
 		NO_STRTOUMAX = YesPlease
+		GIT_TEST_CMP = cmp
 	endif
 	ifeq ($(uname_R),5.8)
 		NO_UNSETENV = YesPlease
 		NO_SETENV = YesPlease
 		NO_C99_FORMAT = YesPlease
 		NO_STRTOUMAX = YesPlease
+		GIT_TEST_CMP = cmp
 	endif
 	ifeq ($(uname_R),5.9)
 		NO_UNSETENV = YesPlease
 		NO_SETENV = YesPlease
 		NO_C99_FORMAT = YesPlease
 		NO_STRTOUMAX = YesPlease
+		GIT_TEST_CMP = cmp
 	endif
 	INSTALL = /usr/ucb/install
 	TAR = gtar
@@ -903,6 +906,7 @@ ifeq ($(uname_S),AIX)
 	else
 		PTHREAD_LIBS = -lpthread
 	endif
+	GIT_TEST_CMP = cmp
 endif
 ifeq ($(uname_S),GNU)
 	# GNU/Hurd
@@ -957,6 +961,7 @@ ifeq ($(uname_S),HP-UX)
 	NO_HSTRERROR = YesPlease
 	NO_SYS_SELECT_H = YesPlease
 	SNPRINTF_RETURNS_BOGUS = YesPlease
+	GIT_TEST_CMP = cmp
 endif
 ifeq ($(uname_S),Windows)
 	GIT_VERSION := $(GIT_VERSION).MSVC

t/Makefile

@@ -3,6 +3,7 @@
 # Copyright (c) 2005 Junio C Hamano
 #
 
+-include ../config.mak.autogen
 -include ../config.mak
 
 #GIT_TEST_OPTS=--verbose --debug