`asv publish` skips results for non-existing commits

[ev-br]

When .asv/results contains a #hash-virtualenv-py3.12.json results file but the commit hash is not present in the benchmarking branch (eg was rebased out), asv publish silently skips this result.
It might be better to still display it even if clicking on the commit in the web view produces 404 link.

My use case is running benchmarks on a cron. So I thought I'd just point asv to a branch, clone main to this branch and push an empty commit. This is not very hard to work around now that I know that asv expects a commit to be present :-). However it would be a little less mysterious if there was always a point on a graph from asv publish for every json result in .asv/results.