fast-import: only allow cat-blob requests where it makes sense

In commit 777f80d7429b ("fast-import: Allow cat-blob requests at
arbitrary points in stream", 2010-11-28), fast-import started allowing
cat-blob commands to appear on the start of any line except in the
middle of a "data" command.  It could be in the middle of various
directives that were part of a tag command, or in the middle of
checkpoints or progresses (each of which allow an optional second empty
newline), or even immediately after the mark command of a blob before
the data directive appeared (raising the question of what if it used the
mark for the blob that just barely appeared in the stream that we do not
yet have the data for).  None of these locations make any sense as
places to put cat-blob requests.

The purpose of this change as stated in that commit message was to

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

However, that can be achieved by simply allowing cat-blob requests to
appear whenever a filemodify directive is allowed.  Further, it avoids
setting a bad precedent for other commands to follow (e.g. get-mark); a
precedent which caused parsing problems in corner cases.

Technically, inline filemodify directives add a slight wrinkle in that
frontends might want to have cat-blob directives appear after the start
of the filemodify and before the data directive contained within it.  I
think it would have been better to disallow such a case (it would be
trivial to use cat-blob before the filemodify instead), but since there
is evidence this was used, for backwards compatibility let's support
that case too.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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