Getting a commit sha1 from fast-import in a remote-helper

Hi,

For some reason, I need to know the sha1 corresponding to some marks
I'm putting in a fast-import stream. Unfortunately, this does not appear
to be possible.
- I'd rather not require a checkpoint to export marks each time I need
  such a sha1, and I'd rather not do that work that requires them after
  all the marks have been created (although currently, I'm forced to).
- fast-import's cat-blob only allows to cat blobs, which, well, is kind
  of in its name; how about adding an equivalent to the git-cat-file
  command?
- fast-import's `ls` command documentation about its output format
  mentions that the output may contain commits, so I tried the trick of
  creating a tree with commits, but fast-import then fails with:
    fatal: Not a blob (actually a commit)
  which I totally understand, but then I wonder why the documentation
  mentions it and how one would get a tree containing references to
  commits. I guess the documentation should be fixed.

So, while there's a possible solution with an equivalent to the git-cat-file command if that'd be accepted, that's also overkill for my need, which is to simply get the sha1 corresponding to a mark. Would you consider a patch adding a command that allows such a resolution?

Cheers,

Mike