Add test for #5881.

test/A.txt

@@ -0,0 +1 @@
+this is a

test/B.txt

@@ -0,0 +1 @@
+here is b

test/C.txt

@@ -0,0 +1 @@
+and this is c

test/D.txt

@@ -0,0 +1 @@
+here's d

test/command/5881.md

@@ -0,0 +1,14 @@
+```
+% pandoc -t markdown -s -H D.txt -d defaults1.yaml -d defaults2.yaml
+Ok
+^D
+here's d
+
+this is a
+
+here is b
+
+and this is c
+
+Ok
+```

test/defaults1.yaml

@@ -0,0 +1 @@
+include-in-header: A.txt

test/defaults2.yaml

@@ -0,0 +1,4 @@
+include-in-header:
+- B.txt
+- C.txt
+