config: complain about --local outside of a git repo

config: complain about --local outside of a git repo

The "--local" option instructs git-config to read or modify
the repository-level config. This doesn't make any sense if
you're not actually in a repository.

Older versions of Git would blindly try to read or write
".git/config". For reading, this would result in a quiet
failure, since there was no config to read (and thus no
matching config value). Writing would generally fail
noisily, since ".git" was unlikely to exist. But since
b1ef400ee (setup_git_env: avoid blind fall-back to ".git",
2016-10-20), we catch this in the call to git_pathdup() and
die with an assertion.

Dying is the right thing to do, but we should catch the
problem early and give a more human-friendly error message.

Note that even without --local, git-config will sometimes
default to using local repository config (e.g., when
writing). These cases are already protected by similar
checks, and covered by a test in t1308.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>