[bug?] checkout -m doesn't work without a base version

Hi,

If during a merge I've resolved conflicts in foo.c but want to start
over with foo.c to resolve them differently, I can say "git checkout
-m foo.c" to restore it to its un-resolved state.

But this only works if there's a base version; if foo.c was added in
each branch, we get:

   error: path 'foo.c' does not have all three versions

Git didn't need all three versions to create the original conflicted
file, so why would it need them to recreate it?

(The message is the same if I explicitly tell Git I don't want diff3
via "git checkout --conflict=merge foo.c".)

If this is considered a bug worth fixing I'll write a test that it
fails; if it's expected behavior I think the docs should mention
that.

Thanks,

Pete Harlan
pgit@pcharlan.com