Two cleanups around 'prefetch' refs

Here are a couple things that caught my eye during a recent evaluation of the maintenance feature:

1. 'refs/prefetch/' refs show up in 'git log' decorations. Auto-hide these.
2. t7900-maintenance.sh had some scary warnings that end up being unimportant.

This is based on 'master' at 66e871b (The third batch, 2021-01-15).

Update in v2: deleting refs more safely for alternate ref backends. (Thanks, Taylor!)

Thanks, -Stolee

Derrick Stolee (2):
maintenance: set log.excludeDecoration durin prefetch
t7900: clean up some broken refs

 builtin/gc.c           |  6 ++++++
 t/t7900-maintenance.sh | 33 ++++++++++++++++++++++++++++++++-
 2 files changed, 38 insertions(+), 1 deletion(-)

base-commit: 66e871b6647ffea61a77a0f82c7ef3415f1ee79c
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-838%2Fderrickstolee%2Fprefetch-refs-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-838/derrickstolee/prefetch-refs-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/838

Range-diff vs v1:

 1:  5b2ce9049a6 = 1:  5b2ce9049a6 maintenance: set log.excludeDecoration durin prefetch
 2:  616b73a6556 ! 2:  35038dfd037 t7900: clean up some broken refs

@@ Commit message

All that is left is to clean up the tests in t7900-maintenance.sh to remove these tags and refs that are not being repacked for the
- incremental-repack tests.
+ incremental-repack tests. Use update-ref to ensure this works with all
+ ref backends.

+ Helped-by: Taylor Blau <[email protected]>

## t/t7900-maintenance.sh ##
@@ t/t7900-maintenance.sh: test_expect_success 'incremental-repack task' '
^HEAD~1
EOF
+
++ # Delete refs that have not been repacked in these packs.
++ git for-each-ref --format="delete %(refname)" \
++ refs/prefetch refs/tags >refs &&
++ git update-ref --stdin <refs &&
++
+ # Replace the object directory with this pack layout.
-+ # However, it does not include all objects from the remotes.
-+ rm -rf .git/refs/prefetch &&
-+ rm -rf .git/refs/tags &&
rm -f $packDir/pack-* &&
rm -f $packDir/loose-* &&
ls $packDir/*.pack >packs-before &&

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