xdl_change_compact(): only use heuristic if group can't be matched

xdl_change_compact(): only use heuristic if group can't be matched

If the changed group of lines can be matched to a group in the other
file, then that positioning should take precedence over the compaction
heuristic.

The old code tried the heuristic unconditionally, which cost redundant
effort and also was broken if the matching code had already shifted the
group higher than the blank line.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>