t5510-fetch.sh: demonstrate fetch.writeCommitGraph bug

While dogfooding, Johannes found a bug in the fetch.writeCommitGraph
config behavior. His example initially happened during a clone with
--recurse-submodules, we found that this happens with the first fetch
after cloning a repository that contains a submodule:

	$ git clone <url> test
	$ cd test
	$ git -c fetch.writeCommitGraph=true fetch origin
	Computing commit graph generation numbers: 100% (12/12), done.
	BUG: commit-graph.c:886: missing parent <hash1> for commit <hash2>
	Aborted (core dumped)

In the repo I had cloned, there were really 60 commits to scan, but
only 12 were in the list to write when calling
compute_generation_numbers(). A commit in the list expects to see a
parent, but that parent is not in the list.

A follow-up will fix the bug, but first we create a test that
demonstrates the problem. This test must be careful about an existing
commit-graph file, since GIT_TEST_COMMIT_GRAPH=1 will cause the repo we
are cloning to already have one. This then prevents the incremtnal
commit-graph write during the first 'git fetch'.

Helped-by: Jeff King <[email protected]>
Helped-by: Johannes Schindelin <[email protected]>
Helped-by: Szeder Gábor <[email protected]>
Signed-off-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

t/t5510-fetch.sh

@@ -583,6 +583,22 @@ test_expect_success 'fetch.writeCommitGraph' '
 	)
 '
 
+test_expect_failure 'fetch.writeCommitGraph with submodules' '
+	git clone dups super &&
+	(
+		cd super &&
+		git submodule add "file://$TRASH_DIRECTORY/three" &&
+		git commit -m "add submodule"
+	) &&
+	git clone "super" super-clone &&
+	(
+		cd super-clone &&
+		rm -rf .git/objects/info &&
+		git -c fetch.writeCommitGraph=true fetch origin &&
+		test_path_is_file .git/objects/info/commit-graphs/commit-graph-chain
+	)
+'
+
 # configured prune tests
 
 set_config_tristate () {