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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>