Is fetch.writeCommitGraph (and thus features.experimental) meant to work in the presence of shallow clones?

Hi,

I was building a version of git for internal use, and thought I'd try turning on features.experimental to get more testing of it. The following test error in the testsuite scared me, though:

t5537.9 (fetch --update-shallow):

...
+ git fetch --update-shallow ../shallow/.git refs/heads/*:refs/remotes/shallow/*
remote: Enumerating objects: 18, done.
remote: Counting objects: 100% (18/18), done.
remote: Compressing objects: 100% (6/6), done.
remote: Total 16 (delta 0), reused 6 (delta 0), pack-reused 0
Unpacking objects: 100% (16/16), 1.16 KiB | 1.17 MiB/s, done.
From ../shallow/
 * [new branch]      master     -> shallow/master
 * [new tag]         heavy-tag  -> heavy-tag
 * [new tag]         light-tag  -> light-tag
error: Could not read ac67d3021b4319951fb176469d7732e6914530c5
error: Could not read ac67d3021b4319951fb176469d7732e6914530c5
error: Could not read ac67d3021b4319951fb176469d7732e6914530c5
fatal: unable to parse commit ac67d3021b4319951fb176469d7732e6914530c5

Passing -c fetch.writeCommitGraph=false to the fetch command in that test makes it pass.

There were also a couple other tests that failed with features.experimental=true (in t5500), but those weren't scary -- they were just checking exact want/have lines and features.experimental is intended to change those. This test from t5537 was the only one that showed some unexpected fatal error.

Thanks, Elijah