t7900: clean up some broken refs

The tests for the 'prefetch' task create remotes and fetch refs into
'refs/prefetch/<remote>/' and tags into 'refs/tags/'. These tests use
the remotes to create objects not intended to be seen by the "local"
repository.

In that sense, the incrmental-repack tasks did not have these objects
and refs in mind. That test replaces the object directory with a
specific pack-file layout for testing the batch-size logic. However,
this causes some operations to start showing warnings such as:

 error: refs/prefetch/remote1/one does not point to a valid object!
 error: refs/tags/one does not point to a valid object!

This only shows up if you run the tests verbosely and watch the output.
It caught my eye and I _thought_ that there was a bug where 'git gc' or
'git repack' wouldn't check 'refs/prefetch/' before pruning objects.
That is incorrect. Those commands do handle 'refs/prefetch/' correctly.

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. Use update-ref to ensure this works with all
ref backends.

Helped-by: Taylor Blau <[email protected]>
Signed-off-by: Derrick Stolee <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

t/t7900-maintenance.sh

@@ -256,6 +256,13 @@ test_expect_success 'incremental-repack task' '
 	HEAD
 	^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.
 	rm -f $packDir/pack-* &&
 	rm -f $packDir/loose-* &&
 	ls $packDir/*.pack >packs-before &&