pack-objects: fix error when packing same pack twice

When passed the same packfile twice via `--stdin-packs` we return an
error that the packfile supposedly was not found. This is because when
reading packs into the list of included or excluded packfiles, we will
happily re-add packfiles even if they are part of the lists already. And
while the list can now contain duplicates, we will only set the `util`
pointer of the first list entry to the `packed_git` structure. We notice
that at a later point when checking that all list entries have their
`util` pointer set and die with an error.

While this is kind of a nonsensical request, this scenario can be hit
when doing geometric repacks. When a repository is connected to an
alternate object directory and both have the exact same packfile then
both would get added to the geometric sequence. And when we then decide
to perform the repack, we will invoke git-pack-objects(1) with the same
packfile twice.

Fix this bug by removing any duplicates from both the included and
excluded packs.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5331-pack-objects-stdin.sh

@@ -7,6 +7,12 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
+packed_objects () {
+	git show-index <"$1" >tmp-object-list &&
+	cut -d' ' -f2 tmp-object-list | sort &&
+	rm tmp-object-list
+ }
+
 test_expect_success 'setup for --stdin-packs tests' '
 	git init stdin-packs &&
 	(
@@ -142,4 +148,25 @@ test_expect_success '--stdin-packs with broken links' '
 	)
 '
 
+test_expect_success 'pack-objects --stdin with duplicate packfile' '
+	test_when_finished "rm -fr repo" &&
+
+	git init repo &&
+	(
+		cd repo &&
+		test_commit "commit" &&
+		git repack -ad &&
+
+		{
+			basename .git/objects/pack/pack-*.pack &&
+			basename .git/objects/pack/pack-*.pack
+		} >packfiles &&
+
+		git pack-objects --stdin-packs generated-pack <packfiles &&
+		packed_objects .git/objects/pack/pack-*.idx >expect &&
+		packed_objects generated-pack-*.idx >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done