deprecate core.statinfo at Git 2.0 boundary

deprecate core.statinfo at Git 2.0 boundary

c08e4d5b5cfa (Enable minimal stat checking, 2013-01-22) advertised
the configuration variable core.checkstat in the documentation and
its log message, but the code expected core.statinfo instead.

For now, add core.checkstat, and warn people who have core.statinfo
in their configuration file that we will remove it in Git 2.0.

Noticed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>