Pushing and fetching sha1s directly

[Finn Arne Gangstad <finnag@pvv.org>]

Currently there seems to be no way of pusing a sha1 directly, or to
fetch a sha1 directly. When working with submodules, it is convenient
to be able to work with detached HEADs, so it would be good if this
could be supported also by fetch and push.

What would be a resonable syntax for this? I'm thinking something like
git fetch --sha1 <repository> <sha1>
git push --sha1 <repository> <sha1>

Where <sha1> could really be a <commit> I guess, but the option name
"--commit" seems wrong somehow. Another option is to extend refspecs
so sha1s can be allowed in there directly, so this would just work:

git fetch <repository> <sha1>

What do you prefer?

- Finn Arne