merge: Make '--log' an integer option for number of shortlog entries

merge: Make '--log' an integer option for number of shortlog entries

Change the command-line '--log' option from a boolean option to an
integer option, and parse the optional integer provided on the
command-line into the 'shortlog_len' variable. Also update the
documentation accordingly.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Reported-by: Yaroslav Halchenko <debian@onerussian.com>
Thanks-to: Jonathan Nieder <jrnieder@gmail.com>
Thanks-to: Johannes Sixt <j.sixt@viscovery.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>