pretty: make it easier to add new formats

pretty: make it easier to add new formats

As the first step towards creating aliases, we make it easier to add new
formats to the list of builtin formats. To do this, we move the
initialization of the formats array into a new function,
setup_commit_formats(), which we can easily extend later. Then, rather
than looping through only the list of known formats, we make a more
generic find_commit_format function, which will return the commit format
whose name is the shortest which is prefixed with the passed-in sought
format, the same rules which were more-or-less hard-coded in before.

Signed-off-by: Will Palmer <wmpalmer@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>