commit: drop uses of get_cached_commit_buffer()

commit: drop uses of get_cached_commit_buffer()

The "--show-all" revision option shows UNINTERESTING
commits. Some of these commits may be unparsed when we try
to show them (since we may or may not need to walk their
parents to fulfill the request).

Commit 3131b71301 (Add "--show-all" revision walker flag for
debugging, 2008-02-09) resolved this by just skipping
pretty-printing for commits without their object contents
cached, saying:

  Because we now end up listing commits we may not even have been parsed
  at all "show_log" and "show_commit" need to protect against commits
  that don't have a commit buffer entry.

That was the easy fix to avoid the pretty-printer segfaulting,
but:

  1. It doesn't work for all formats. E.g., --oneline
     prints the oid for each such commit but not a trailing
     newline, leading to jumbled output.

  2. It only affects some commits, depending on whether we
     happened to parse them or not (so if they were at the
     tip of an UNINTERESTING starting point, or if we
     happened to traverse over them, you'd see more data).

  3. It unncessarily ties the decision to show the verbose
     header to whether the commit buffer was cached. That
     makes it harder to change the logic around caching
     (e.g., if we could traverse without actually loading
     the full commit objects).

These days it's safe to feed such a commit to the
pretty-print code. Since be5c9fb904 (logmsg_reencode: lazily
load missing commit buffers, 2013-01-26), we'll load it on
demand in such a case. So let's just always show the verbose
headers.

This does change the behavior of plumbing, but:

  a. The --show-all option was explicitly introduced as a
     debugging aid, and was never documented (and has rarely
     even been mentioned on the list by git devs).

  b. Avoiding the commits was already not deterministic due
     to (2) above. So the caller might have seen full
     headers for these commits anyway, and would need to be
     prepared for it.

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