The patch extends git config --file interface to allow read config from
stdin.

Editing stdin or setting value in stdin is an error.

Include by absolute path is allowed in stdin config, but not by relative
path.
Signed-off-by: NKirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>