pretty_print_commit() has a bunch of rarely-used arguments, and
introducing more of them requires yet another update of all the call
sites.  Refactor most of them into a struct to make future extensions
easier.

The ones that stay "plain" arguments were chosen on the grounds that
all callers put real arguments there, whereas some callers have 0/NULL
for all arguments that were factored into the struct.

We declare the struct 'const' to ensure none of the callers are bitten
by the changed (no longer call-by-value) semantics.
Signed-off-by: NThomas Rast <trast@student.ethz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>