diff: fix reply indicator
diff: fix reply indicator

The reply indicator for individual messages was always empty, and the
reply indicator for the cover letter showed comments for the entire
series.  (This happened to be correct for the two cases I used for
testing...).  Use an EXISTS subquery just like the series view does
to fix it.

Signed-off-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>

patchew-project/patchew,patchew-project/patchew,famz/patchew,famz/patchew,patchew-project/patchew,patchew-project/patchew,famz/patchew,famz/patchew