Merge branch 'cr/git-work-tree-sans-git-dir'

These days, "git --work-tree=there cmd" without specifying an
explicit --git-dir=here will do the usual discovery, but we had a
description of older behaviour in the documentation.

* cr/git-work-tree-sans-git-dir:
git.txt: remove stale comment regarding GIT_WORK_TREE

diff --cc Documentation/git.txt
Simple merge