Since b9605bc4

 ("config: only read .git/config from configured
repos", 2016-09-12), we do not read from ".git/config" unless we
know we are in a repository.  "git archive" however didn't do the
repository discovery and instead relied on the old behaviour.

Teach the command to run a "gentle" version of repository discovery
so that local configuration variables are honoured.

[jc: stole tests from peff]
Signed-off-by: Junio C Hamano <gitster@pobox.com>