builtin-fsck: fix off by one head count

builtin-fsck: fix off by one head count

According to the man page, if "git fsck" is passed one or more heads, it
should verify connectivity and validity of only objects reachable from the
heads it is passed.

However, since 5ac0a20 (Make builtin-fsck.c use parse_options.,
2007-10-15) the command behaved as if no heads were passed, when given
only one argument.

Signed-off-by: Christian Couder <redacted>
Signed-off-by: Junio C Hamano <redacted>