fast-import: Allow cat-blob requests at arbitrary points in stream

fast-import: Allow cat-blob requests at arbitrary points in stream

The new rule: a "cat-blob" can be inserted wherever a comment is
allowed, which means at the start of any line except in the middle of
a "data" command.

This saves frontends from having to loop over everything they want to
commit in the next commit and cat-ing the necessary objects in
advance.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>