Merge branch 'maint'

By Jens Lehmann (1) and Johannes Sixt (1)
* maint:
  Consistently use "superproject" instead of "supermodule"
  t3404: begin "exchange commits with -p" test with correct preconditions

diff --cc t/t3404-rebase-interactive.sh
Simple merge

diff --cc unpack-trees.c
Simple merge