Temporarily set -x to reveal git tag failure

This temporarily instruments a fixture script to reveal how the
`access::generation_numbers_overflow_is_handled_in_chained_graph`
test fails when `GIX_TEST_IGNORE_ARCHIVES=1` is set: the fixture
script `generation_number_overflow.sh` runs, and it sets the
timestamp `@4147483646` on a commit, which `git` permits. But then
`git tag` rejects this timestamp as being out of range. On stderr:

    + commit future-1 '@4147483646 +0000'
    + local message=future-1
    + local 'date=@4147483646 +0000'
    + local file=future-1.t
    + echo future-1
    + git add -- future-1.t
    + '[' -n '@4147483646 +0000' ']'
    + export 'GIT_COMMITTER_DATE=@4147483646 +0000'
    + GIT_COMMITTER_DATE='@4147483646 +0000'
    + git commit -m future-1
    + git tag future-1
    fatal: Timestamp too large for this system: 4147483646

The nature of the problem is *partially* illuminated by the
following experiment on a 32-bit Debian 12 system:

    $ git init repo
    Initialized empty Git repository in /home/ek/src/repo/.git/
    $ cd repo
    $ touch a
    $ git add a
    $ GIT_COMMITTER_DATE='@4147483646 +0000' git commit -m 'Initial commit'
    [main (root-commit) 99b1de4] Initial commit
     1 file changed, 0 insertions(+), 0 deletions(-)
     create mode 100644 a
    $ git log --pretty=fuller
    commit 99b1de4e0610ff715e784bd4089dc315e8071332 (HEAD -> main)
    Author:     Eliah Kagan <degeneracypressure@gmail.com>
    AuthorDate: Sat Nov 23 01:05:12 2024 -0500
    Commit:     Eliah Kagan <degeneracypressure@gmail.com>
    CommitDate: Thu Jan 1 00:00:00 1970 +0000

        Initial commit
    $ git tag t
    fatal: Timestamp too large for this system: 4147483646
    $ git tag -a t
    fatal: Timestamp too large for this system: 4147483646

The timestamp is accepted when committing, and written as actor
metadata for the committer. But is is not (or at least not always)
interpreted as intended: as `git log` shows it, it has wrapped
around to 1970. Even though the commit was created successfully,
attempting to make a tag that points to it fails. This happens both
for a lightweight tag (as in the fixture script) and an annotated
tag, so making it a tag object does not seem to be a workaround.

Author: EliahKagan

gix-commitgraph/tests/fixtures/generation_number_overflow.sh

@@ -36,6 +36,7 @@ FUTURE_DATE="@4147483646 +0000"
 git init
 git config commitGraph.generationVersion 2
 
+set -x
 commit future-1 "$FUTURE_DATE"
 commit old-1 "$UNIX_EPOCH_ZERO"
 git commit-graph write --reachable