codeintel: Validate input commit
Created by: efritz
Closes https://github.com/sourcegraph/sourcegraph/issues/14052.
This reverts the change in https://github.com/sourcegraph/sourcegraph/pull/14005, then strengthens the validation on the input commit.
In short, we pass the repo name and commit to gitserver for resolution. If gitserver is currently cloning the repo, we can't determine what the correct commit is. If this happens to be a symbolic name, it will fail to be put into the database. This error message helps with this case.
Note to reviewer: This is weenie hut junior mode if reviewed by commit.