[PATCH v4 1/2] t5510-fetch.sh: demonstrate fetch.writeCommitGraph bug

From: Derrick Stolee <dstolee@microsoft.com>

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 <peff@peff.net>
Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: Szeder Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 t/t5510-fetch.sh | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)