Merge branch 'cp/git-flush-is-an-env-bool' into maint-2.43

Unlike other environment variables that took the usual
true/false/yes/no as well as 0/1, GIT_FLUSH only understood 0/1,
which has been corrected.

* cp/git-flush-is-an-env-bool:
  write-or-die: make GIT_FLUSH a Boolean environment variable

diff --cc Documentation/git.txt
Simple merge