Merge branch 'as/merge-attr-sleep' into maint

Fix for a racy false-positive test failure.

* as/merge-attr-sleep:
t6026: clarify the point of "kill $(cat sleep.pid)"
t6026: ensure that long-running script really is
Revert "t6026-merge-attr: don't fail if sleep exits early"
Revert "t6026-merge-attr: ensure that the merge driver was called"
t6026-merge-attr: ensure that the merge driver was called
t6026-merge-attr: don't fail if sleep exits early