maintenance: set log.excludeDecoration durin prefetch

The 'prefetch' task fetches refs from all remotes and places them in the
refs/prefetch/<remote>/ refspace. As this task is intended to run in the
background, this allows users to keep their local data very close to the
remote servers' data while not updating the users' understanding of the
remote refs in refs/remotes/<remote>/.

However, this can clutter 'git log' decorations with copies of the refs
with the full name 'refs/prefetch/<remote>/<branch>'.

The log.excludeDecoration config option was added in a6be5e67 (log: add
log.excludeDecoration config option, 2020-05-16) for exactly this
purpose.

Ensure we set this only for users that would benefit from it by
assigning it at the beginning of the prefetch task. Other alternatives
would be during 'git maintenance register' or 'git maintenance start',
but those might assign the config even when the prefetch task is
disabled by existing config. Further, users could run 'git maintenance
run --task=prefetch' using their own scripting or scheduling. This
provides the best coverage to automatically update the config when
valuable.

It is improbable, but possible, that users might want to run the
prefetch task _and_ see these refs in their log decorations. This seems
incredibly unlikely to me, but users can always opt-in on a
command-by-command basis using --decorate-refs=refs/prefetch/.

Test that this works in a few cases. In particular, ensure that our
assignment of log.excludeDecoration=refs/prefetch/ is additive to other
existing exclusions. Further, ensure we do not add multiple copies in
multiple runs.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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