During a pre-commit hook (actually a ref-update in Gerrit) the repository is in a detached HEAD mode (witch is fine).
Doing a git clone path/to/my/repo does clone/fetch this commit, but I'd like to just do a git fetch and avoid cloning my repository each time there is a commit (for various reasons like avoiding to change file time stamps each time).
Doing a git fetch --all on a previously cloned repository (like above) does not fetch this detached head commit. Note that I do have the commit's SHA1 but a git checkout commit-sha1-here doesn't work either since it hasn't been fetched.
I do know it's by design that only remote branches are fetched, but since clone can fetch detached heads, isn't there a way to update my cloned repository to retrieve them later? I'd like to avoid having to create a temporary branch each time just for that.
The final goal is to have an updated work-tree of a bare repository (detached HEAD).
Answering my own question here.
git pull of a detached HEAD seems not allowed by design but a git push of that detached HEAD is possible (i.e., instead of fetching from the receiving side you push from the sending side):
$ git push <repository> +<sha1>:refs/remotes/origin/master
Then from the repository receiving it:
$ git checkout <sha1> # same as origin/master
Note: Pushing to refs/remotes/origin/master instead of pushing directly to master is disallowed by default if it's the current branch or a non-base repository, so we push to the origin reference.