http-fetch: don't crash when parsing packfile without a repo

http-fetch: don't crash when parsing packfile without a repo

The git-http-fetch(1) command accepts a `--packfile=` option, which
allows the user to specify that it shall fetch a specific packfile,
only. The parameter here is the hash of the packfile, which is specific
to the object hash used by the repository. This requirement is implicit
though via our use of `parse_oid_hex()`, which internally uses
`the_repository`.

The git-http-fetch(1) command allows for there to be no repository
though, which only exists such that we can show usage via the "-h"
option. In that case though, starting with c8aed5e8da (repository: stop
setting SHA1 as the default object hash, 2024-05-07), `the_repository`
does not have its object hash initialized anymore and thus we would
crash when trying to parse the object ID outside of a repository.

Fix this issue by dying immediately when we see a "--packfile="
parameter when outside a Git repository. This is not a functional
regression as we would die later on with the same error anyway.

Add a test to detect the segfault. We use the "nongit" function to do
so, which we need to allow-list in `test_must_fail ()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>