[PATCHv3 0/5] Simple fixes to t7406

This series started as a simple single-line fix, but folks keep noticing other problems with t7406 while reading my patches. So, changes noted below and I have a challenge at the end.

Changes since v2:
  - Two new patches inserted into this series (3/5 and 4/5):
    - Prefer test_* helper functions to 'test -[feds]' (see patch 3; this
      is my attempt to remove easy answers from the challenge below)
    - Fix other places in the testfile using test_must_fail for non-git
      commands (see patch 4; suggested by SZEDER)
  - Avoid losing the check of git's exit status in a command substituion,
    improve potential error message for a test while at it (part of patch 2;
    suggested by SZEDER)
  - Update the commit message for the final commit to match the updated
    code (Pointed out by Martin)

Since folks like to notice other problems with t7406 while reading my patches, here's a challenge:

  Find something *else* wrong with t7406 that neither I nor any of the
  reviewers so far have caught that could be fixed.

    - You get bonus points if that thing is in the context region for
      one of my five patches.
    - Extra bonus points if the thing needing fixing was on a line I
      changed.
    - You win outright if it's something big enough that I give up and
      request to just have my series merged as-is and punt your
      suggested fixes down the road to someone else.

  (Note: If I flubbed something in v3, pointing it out doesn't count
   as finding "something else", but do please still point it out.)

:-)

Elijah Newren (5):
  t7406: simplify by using diff --name-only instead of diff --raw
  t7406: avoid having git commands upstream of a pipe
  t7406: prefer test_* helper functions to test -[feds]
  t7406: avoid using test_must_fail for commands other than git
  t7406: fix call that was failing for the wrong reason

 t/t7406-submodule-update.sh | 37 +++++++++++++++++++++++--------------
 1 file changed, 23 insertions(+), 14 deletions(-)