Provide a more meaningful error-message for `builtins.fetchGit` if a revision can't be checked out

A common pitfall when using e.g. `builtins.fetchGit` is the `fatal: not
a tree object`-error when trying to fetch a revision of a git-repository
that isn't on the `master` branch and no `ref` is specified.

In order to make clear what's the problem, I added a simple check
whether the revision in question exists and if it doesn't a more
meaningful error-message is displayed:

```
nix-repl> builtins.fetchGit { url = "https://github.com/owner/myrepo"; rev = "<commit not on master>"; }
moderror: --- Error -------------------------------------------------------------------- nix
Cannot find Git revision 'bf1cc5c648e6aed7360448a3745bb2fe4fbbf0e9' in ref 'master' of repository 'https://gitlab.com/Ma27/nvim.nix'! Please make sure that the rev exists on the ref you've specified or add allRefs = true; to fetchGit.
```

Closes #2431

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