cat-file: dies with --batch if reflog entry is out of bounds

["Kristoffer Haugsbakk" <kristofferhaugsbakk@fastmail.com>]

    $ git cat-file --batch
    HEAD@{7}
    <commit>
    branch@{20}
    fatal: log for 'branch' only has 5 entries
    [died]

This is a known limitation.

It used to just die without any flags until recently in 393bbb21
(object-name: make get_oid quietly return an error, 2025-06-12).[1] That
added a `GET_OID_GENTLY` flag which is respected here. If that is set
it returns normally from the function with an error.

Before that I was looking at `builtin/cat-file.c` and guessing that
`GET_OID_GENTLY` would be the thing that made `--batch` loop on. But it
seems to be the lack of `GET_OID_ONLY_TO_DIE`; that flag seems to be
used when you do a one object lookup like `git cat-file commit
HEAD`. And it is indeed not set for the batch modes.

`GET_OID_GENTLY` from commit 393bbb21 was added it seems so that a
commit in the same series could try to look up a reflog entry in
`builtin/stash.c` and give its own error message. And go to `cleanup` as
well. It’s the only user/usage.

I’m pretty new to git-cat-file(1) but it seems very consistent in
returning “missing” for object names that don’t exist, “little red
riding hood” and malformed rev expressions. Is this the only
could-be-an-object input it will die on?

[1]: https://lore.kernel.org/git/20250508234458.3665894-2-sandals@crustytoothpaste.net/#t

-- 
Kristoffer Haugsbakk
code at khaugsbakk.name