diff --git a/Makefile b/Makefile
index 668dbc96ca..c8cc9e280a 100644
--- a/Makefile
+++ b/Makefile
@@ -1374,6 +1374,10 @@ ifdef USE_NED_ALLOCATOR
        COMPAT_OBJS += compat/nedmalloc/nedmalloc.o
 endif
 
+ifdef GIT_TEST_CMP_USE_COPIED_CONTEXT
+	export GIT_TEST_CMP_USE_COPIED_CONTEXT
+endif
+
 ifeq ($(TCLTK_PATH),)
 NO_TCLTK=NoThanks
 endif
diff --git a/t/Makefile b/t/Makefile
index 93a64750eb..25c559bb49 100644
--- a/t/Makefile
+++ b/t/Makefile
@@ -6,14 +6,10 @@
 -include ../config.mak
 
 #GIT_TEST_OPTS=--verbose --debug
-GIT_TEST_CMP ?= $(DIFF)
 SHELL_PATH ?= $(SHELL)
 TAR ?= $(TAR)
 RM ?= rm -f
 
-# Make sure test-lib.sh uses make's value of GIT_TEST_CMP
-export GIT_TEST_CMP
-
 # Shell quote;
 SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))
 
diff --git a/t/test-lib.sh b/t/test-lib.sh
index c582964b0d..a290011fa7 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -63,7 +63,16 @@ export GIT_MERGE_VERBOSITY
 export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
 export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export EDITOR
-GIT_TEST_CMP=${GIT_TEST_CMP:-diff -u}
+
+if test -z "$GIT_TEST_CMP"
+then
+	if test -n "$GIT_TEST_CMP_USE_COPIED_CONTEXT"
+	then
+		GIT_TEST_CMP="$DIFF -c"
+	else
+		GIT_TEST_CMP="$DIFF -u"
+	fi
+fi
 
 # Protect ourselves from common misconfiguration to export
 # CDPATH into the environment