Explain "Not a git repository: '.git'".

Explain "Not a git repository: '.git'".

Andy Parkins noticed that the error message some "whole tree"
oriented commands emit is stated misleadingly when they refused
to run from a subdirectory.

We could probably allow some of them to work from a subdirectory
but that is a semantic change that could have unintended side
effects, so let's start at first by rewording the error message
to be easier to read without doing anything else to be safe.

Signed-off-by: Junio C Hamano <redacted>