By using 'git commit-graph verify --shallow' we can ensure that
the file we just wrote is valid. This is an extra safety precaution
that is faster than our 'write' subcommand. In the rare situation
that the newest layer of the commit-graph is corrupt, we can "fix"
the corruption by deleting the commit-graph-chain file and rewrite
the full commit-graph as a new one-layer commit graph. This does
not completely prevent _that_ file from being corrupt, but it does
recompute the commit-graph by parsing commits from the object
database. In our use of this step in Scalar and VFS for Git, we
have only seen this issue arise because our microsoft/git fork
reverted 43d3561 ("commit-graph write: don't die if the existing
graph is corrupt" 2019-03-25) for a while to keep commit-graph
writes very fast. We dropped the revert when updating to v2.23.0.
The verify still has potential for catching corrupt data across
the layer boundary: if the new file has commit X with parent Y
in an old file but the commit ID for Y in the old file had a
bitswap, then we will notice that in the 'verify' command.

I'm concerned about having this extra precaution, because it is never
tested (neither here or in a future patch). When you saw this issue
arise, was there ever an instance in which a valid set of commit graph
files turned invalid after this maintenance step? (It seems from your
description that the set was invalid to begin with, so the maintenance
step did not fix it but also did not make it worse. And it does not make
it worse, that seems not too bad to me.)

Other than that, this patch looks good to me.