bashunit init accumulates a dead BASHUNIT_BOOTSTRAP line on every run #1175

Problem

bashunit init comments out any existing BASHUNIT_BOOTSTRAP and appends a
fresh one — unconditionally, even when the value is unchanged. Three runs:

#BASHUNIT_BOOTSTRAP=tests/bootstrap.sh
#BASHUNIT_BOOTSTRAP=tests/bootstrap.sh
BASHUNIT_BOOTSTRAP=tests/bootstrap.sh

The scaffolded files are idempotent, so re-running init is otherwise safe and
people do it. Only .env grows.

.env is also the one file init writes without saying so — the other
three print > Created …, so someone reviewing what init touched before
committing would miss it.

Fix

Skip the rewrite when the setting already has the intended value, and report
each .env outcome:

> Created .env                              # first run
> .env already sets BASHUNIT_BOOTSTRAP      # re-run, unchanged
> Updated .env                              # value differed

Replacing a different value keeps working — that is what the comment-and-append
logic is for.