Merge branch 'dw/diff-no-index-doc'

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