When the user types "git diff" outside a working tree, thinking he
is inside one, the current error message that is a single-liner
"usage: git diff --no-index <path> <path>" may not be sufficient to
make him realize the mistake. Add "Not a git repository" to the
error message when we fell into the "--no-index" mode without an
explicit command line option to instruct us to do so.

* dw/diff-no-index-doc:
  diff --no-index: describe in a separate paragraph
  diff --no-index: clarify operation when not inside a repository