git config: report when trying to modify a non-existing repo config

It is a pilot error to call `git config section.key value` outside of
any Git worktree. The message

error: could not lock config file .git/config: No such file or
directory

is not very helpful in that situation, though. Let's print a helpful
message instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>