help: align the longest command in the command listing

help: align the longest command in the command listing

"longest" is used to determine how many extra spaces we need to print
to keep the command description aligned. For the longest command, we
should print no extra space instead of one, or we'll get unaligned
output like this (notice the "checkout" line):

    grow, mark and tweak your common history
       branch     List, create, or delete branches
       checkout    Switch branches or restore working tree files
       commit     Record changes to the repository
       diff       Show changes between commits, commit and ...
       merge      Join two or more development histories together
       rebase     Reapply commits on top of another base tip
       tag        Create, list, delete or verify a tag ...

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>