tests: diff3: work around missing seq on some systems

* tests/diff3 (seq): Provide a seq replacement function,
since at least AIX, SunOS 5.10, OpenBSD-5.8 lack it.
Reported by Assaf Gordon in https://bugs.gnu.org/24227#8

tests/diff3

@@ -3,6 +3,18 @@
 
 . "${srcdir=.}/init.sh"; path_prepend_ ../src
 
+# Some systems lack seq.
+# A limited replacement for seq: handle 1 or 2 args; increment must be 1
+seq()
+{
+  case $# in
+    1) start=1  final=$1;;
+    2) start=$1 final=$2;;
+    *) echo you lose 1>&2; exit 1;;
+  esac
+  awk 'BEGIN{for(i='$start';i<='$final';i++) print i}' < /dev/null
+}
+
 echo a > a || framework_failure_
 echo b > b || framework_failure_
 echo c > c || framework_failure_