commit.c: check for lock error and return early

commit.c: check for lock error and return early

Move the check for the lock failure to happen immediately after
lock_any_ref_for_update().  Previously the lock and the
check-if-lock-failed was separated by a handful of string
manipulation statements.

Moving the check to occur immediately after the failed lock makes
the code slightly easier to read and makes it follow the pattern of

 try-to-take-a-lock();
 if (check-if-lock-failed) {
    error();
 }

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>