clean up calling conventions for pretty.c functions

clean up calling conventions for pretty.c functions

We have a pretty_print_context representing the parameters
for a pretty-print session, but we did not use it uniformly.
As a result, functions kept growing more and more arguments.

Let's clean this up in a few ways:

  1. All pretty-print pp_* functions now take a context.
     This lets us reduce the number of arguments to these
     functions, since we were just passing around the
     context values separately.

  2. The context argument now has a cmit_fmt field, which
     was passed around separately. That's one less argument
     per function.

  3. The context argument always comes first, which makes
     calling a little more uniform.

This drops lines from some callers, and adds lines in a few
places (because we need an extra line to set the context's
fmt field). Overall, we don't save many lines, but the lines
that are there are a lot simpler and more readable.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>