git-contrib: copypaste-friendly output

Format output in a manner that can be copypasted as-is to NEWS.
That is, with 8 spaces indentation and wrapped at 80 columns.

Before:

$ tools/git-contrib.sh
        Ben Stockett,
        Carl Lei,
        Frantisek Sumsal,
        Gibeom Gwon,
        Hugo Osvaldo Barrera,
        James Hilliard,
        Jan Palus,
        Lennart Poettering,
        Luca Boccassi,
        Luca BRUNO,
        Mike Gilbert,
        nassir90,
        nl6720,
        Raul Tambre,
        Yegor Alexeyev,
        Yu Watanabe,
        Zbigniew Jędrzejewski-Szmek,

After:

        Contributions from: Ben Stockett, Carl Lei, Frantisek Sumsal,
        Gibeom Gwon, Hugo Osvaldo Barrera, James Hilliard, Jan Palus,
        Lennart Poettering, Luca Boccassi, Luca BRUNO, Mike Gilbert,
        nassir90, nl6720, Raul Tambre, Yegor Alexeyev, Yu Watanabe,
        Zbigniew Jędrzejewski-Szmek