Re: [PATCH 2/3] [GSOC] ref-filter: support %(contents) for blob, tree

To extend on this point a bit (even though this is not all that
urgent during the prerelease freeze), conceptually, the %(content)
field is understood in the larger picture like this:

+--- (the whole thing) ----------------------------------------------+
|                                                                    |
| +--- "header" ---------------------------------------------------+ |
| | tree 678c03dca0a26afd746e8c8bb9e4aadc8bf479b1                  | |
| | parent 378c7c6ad48c4ccddf9b534616a0e86f28440bd3                | |
| | author Junio C Hamano [off-list ref] 1621675665 +0900     | |
| | committer Junio C Hamano [off-list ref] 1621675741 +0900  | |
| +----------------------------------------------------------------+ |
|                                                                    |
| +--- "contents" -------------------------------------------------+ |
| |                                                                | |
| | +--- "subject" ----------------------------------------------+ | |
| | | Git 2.32-rc1                                               | | |
| | +------------------------------------------------------------+ | |
| |                                                                | |
| | +--- "body" -------------------------------------------------+ | |
| | | Signed-off-by: Junio C Hamano [off-list ref]          | | |
| | +------------------------------------------------------------+ | |
| |                                                                | |
| +----------------------------------------------------------------+ |
|                                                                    |
+--------------------------------------------------------------------+

Thank you for providing such a complete view. This also means
that the "raw" of contents and tag will contain two parts: "header"
and "contents". But for blobs and trees, they don’t have these things.

Even though %(header), when it is invented, would make perfect sense
for commits and tags, it will never make sense for trees and blobs.
Which means "contents", which is "the whole thing except for the
header part", would not, either.

Although we don’t have a %(header), but in fact we already have fragments
of "%(numparent)", "%(parent)" %(tree)" (see grab_commit_values()) and
"%(tag)"," %(type)","%(object)" (see grab_tag_values()), but they are not
obtained through the "header" part of the raw object buffer.

There is no %(placeholder) to ask for "the whole thing", and that is
what you want to use for cat-file --batch if I am not mistaken, and
adding one would be a good idea.  There is no %(header) yet, either,
but if somebody needs it for their scripts, it is clear where it fits
in the picture.

So I don't know if adding %(header) will cause duplication of functions.

Thanks!
--
ZheNing Hu