merge: do not check argc to determine number of remote heads

merge: do not check argc to determine number of remote heads

To reject merging multiple commits into an unborn branch, we check
argc, thinking that collect_parents() that reads the remaining
command line arguments from <argc, argv> will give us the same
number of commits as its input, i.e. argc.

Because what we really care about is the number of commits, let the
function run and then make sure it returns only one commit instead.

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