Can you allow git fetching specific commits?
From a quick googling, it looks like this isn't a server configuration, but most likely the commit you're looking for just isn't in the remote.
Why are you using this command? Are you using submodules for deployment?
The specific set of commands im looking at are
mkdir -p phabricator_git_repo && cd phabricator_git_repo && git init && git remote add origin https://we.phorge.it/source/phorge.git && git fetch origin b11c6fcacd8bceafc2a2f223037952dc817f4d7f && git reset --hard FETCH_HEAD && rm -rf .git && true
this commit does exist https://we.phorge.it/rPb11c6fcacd8bceafc2a2f223037952dc817f4d7f
we use a build system which fetches the code from a git repo and makes it available to the next stage of the build
From reading the git code, looks like there is a flag to allow this feature - uploadpack.allowReachableSHA1InWant, though I'll need to check that it actually works.
BTW, you'll save some time (and our bandwidth) if you add --depth 1 to the fetch phase.
(We'll also have a GitHub mirror at some point you could fetch from, they support this feature).