bash: support pretty format aliases

bash: support pretty format aliases

Users can have their own pretty format aliases since 8028184 (pretty:
add aliases for pretty formats, 2010-05-02), so let's offer those
after '--pretty=' and '--format=' for 'log' and 'show', too.

Similar to the completion of aliases, this will invoke 'git config'
each time pretty aliases needs to be completed, so changes in pretty.*
configuration will be reflected immediately.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>