Revert "Merge branch 'mt/patch-id-stable' into next"

This reverts commit 0188d44bb409916a5e96cf6d7160e9e9ed064cc5,
reversing changes made to 6b74773678cb408b7edb236752e514fa00488637,
to replace it with a new round to be cooked in 'next' for the
remainder of this cycle.
builtin/show: do not prune by pathspec

By design, "git show commit -- path" is not "git show commit:path", and
there is no reason to change that. But "git show commit -- path" simply
returns nothing at all "most of the time" because it prunes by pathspec
even though it does not walk commits. This is pretty useless.

So, turn off commit pruning (but keep diff limiting of course) so that
"git show commit -- path" shows the commit message and the diff that the
commit introduces to path (filtered by path); only the diff will be
empty "most of the time".

As an intended side effect, users mistaking "git show commit -- path"
for "git show commit:path" are automatically reminded that they asked
git to show a commit, not a blob.

In case the user has specified "--do-walk", assume they want the old
behaviour (prune by default).

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>