Merge branch 'en/t7406-fixes'

Test fixes.

* en/t7406-fixes:
  t7406: avoid using test_must_fail for commands other than git
  t7406: prefer test_* helper functions to test -[feds]
  t7406: avoid having git commands upstream of a pipe
  t7406: simplify by using diff --name-only instead of diff --raw
  t7406: fix call that was failing for the wrong reason

diff --cc t/t7406-submodule-update.sh
Simple merge