The -d option for diff is GNU extension.

--- citus-13.1.0/src/test/regress/bin/test_diff.orig
+++ citus-13.1.0/src/test/regress/bin/test_diff
@@ -10,7 +10,7 @@
 # diff file.out against file_$1.out, also strip out timestamps & file paths
 function create_result()
 {
-	diff -dU2 -w "$SCRIPTPATH/test/file.out" "$SCRIPTPATH/test/file_$1.out" \
+	diff -U2 -w "$SCRIPTPATH/test/file.out" "$SCRIPTPATH/test/file_$1.out" \
 		| sed -E 's/^(\+\+\+|---).+\/([^/]+)\t.+$/\1 \2/' \
 		> "$SCRIPTPATH/test/results/$1.out"
 }
--- citus-13.1.0/src/test/regress/pg_regress_multi.pl.orig
+++ citus-13.1.0/src/test/regress/pg_regress_multi.pl
@@ -159,7 +159,7 @@
 # a bit more context to make it easier to locate failed test sections.
 #
 # Also, ignore whitespace, without this the diffs on windows are unreadable
-$ENV{PG_REGRESS_DIFF_OPTS} = '-dU10 -w';
+$ENV{PG_REGRESS_DIFF_OPTS} = '-U10 -w';
 
 my $plainRegress = "";
 my $isolationRegress = "";
--- citus-13.1.0/src/test/regress/citus_tests/run_test.py.orig
+++ citus-13.1.0/src/test/regress/citus_tests/run_test.py
@@ -268,7 +268,7 @@
 
     os.chdir(REGRESS_DIR)
     os.environ["PATH"] = str(REGRESS_DIR / "bin") + os.pathsep + os.environ["PATH"]
-    os.environ["PG_REGRESS_DIFF_OPTS"] = "-dU10 -w"
+    os.environ["PG_REGRESS_DIFF_OPTS"] = "-U10 -w"
 
     fake_config_args = {
         "--pgxsdir": str(pgxs_path.parent.parent.parent),
--- citus-13.1.0/src/test/regress/Makefile.orig
+++ citus-13.1.0/src/test/regress/Makefile
@@ -14,7 +14,7 @@
 # We do this to be able to mask shard Ids, placement Ids, node ports, etc.
 MAKEFILE_DIR := $(dir $(realpath $(firstword $(MAKEFILE_LIST))))
 export PATH := $(MAKEFILE_DIR)/bin:$(PATH)
-export PG_REGRESS_DIFF_OPTS = -dU10 -w
+export PG_REGRESS_DIFF_OPTS = -U10 -w
 # Use lower isolation test timeout, the 5 minute default is waaay too long for
 # us so we use 60 seconds instead. We should detect blockages very quickly and
 # most queries that we run are also very fast. So fast even that 60 seconds is