check_repository_format_gently(): refuse extensions for old repositories

check_repository_format_gently(): refuse extensions for old repositories

Previously, extensions were recognized regardless of repository format
version.  If the user sets an undefined "extensions" value on a
repository of version 0 and that value is used by a future git version,
they might get an undesired result.

Because all extensions now also upgrade repository versions, tightening
the check would help avoid this for future extensions.

Signed-off-by: Xin Li <delphij@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>