Can I fetch an arbitrary commit by sha1?

From: Christian Halstrick <christian.halstrick@gmail.com>
Date: Oct 2, 2014, 13:57 UTC
Message-ID: <CAENte7htO13s91UJFNzW4aBhsGxE=LpnvaZfce+vqQU5+a-cYg@mail.gmail.com>

I always though during fetch I have to specify a refspec and that a sha1 would not be accepted as a ref. Firing some like 'git fetch origin <sha1>' should be forbidden. But in fact I see that such a fetch command succeeds if you already have that object in your local repo.

My question: is it allowed to fetch sha1's? Shouldn't fetch fail if you try it?

See here:

> git clone -q https://github.com/chalstrick/dondalfi.git
> cd dondalfi
> git ls-remote

From https://github.com/chalstrick/dondalfi.git ce08dcc41104383f3cca2b95bd41e9054a957f5b HEAD af00f4c39bcc8dc29ed8f59a47066d5993c279e4 refs/foo/b1 ...

> git show af00f4c39bcc8dc29ed8f59a47066d5993c279e4

fatal: bad object af00f4c39bcc8dc29ed8f59a47066d5993c279e4

> git fetch origin af00f4c39bcc8dc29ed8f59a47066d5993c279e4

error: no such remote ref af00f4c39bcc8dc29ed8f59a47066d5993c279e4

> git fetch origin refs/foo/b1

remote: Counting objects: 3, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From https://github.com/chalstrick/dondalfi
 * branch            refs/foo/b1 -> FETCH_HEAD

> git fetch origin af00f4c39bcc8dc29ed8f59a47066d5993c279e4

From https://github.com/chalstrick/dondalfi
 * branch            af00f4c39bcc8dc29ed8f59a47066d5993c279e4 -> FETCH_HEAD

Ciao
  Chris