packfile: fix perf regression with many packs

Since 589127caa730 (packfile: move list of packs into the packfile
store, 2025-10-30), there is a performance regression when many
packfiles need to be loaded: `packfile_store_add_pack()` now calls
`packfile_list_remove_internal()` to detect whether the packfile was
_already_ in the list, and if so, move it to the end of the list. This
function linearly scans the existing list before every insertion. Newly
loading N packs therefore has complexity O(N²).

In one reported use case (https://github.com/microsoft/git/issues/970),
N equals 37,815 and caused a slow-down of a simple `git rev-parse
--short HEAD` (which is regularly executed as part of `GIT_PS1`) from
0.4s to 4.5s.

Let's fix this by establishing a fast path for known-new packfiles.

The keen reader will note that there is currently only a single,
"known-new" caller of the `packfile_list_append()` function, and wonder
why not simply remove this check whether the packfile already exists in
the list? Originally, when above-mentioned commit introduced that logic,
there was a second caller in `prepare_midx()`, which would have required
that check, but that caller was removed in 6aff1f25a046 (packfile:
always add packfiles to MRU when adding a pack, 2025-10-30). Still, the
function is declared in a header file, and to avoid any problems with
in-flight or downstream callers, it is safer to extend the signature to
be explicit whether or not to skip that check.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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