Make git-archimport log entries more consistent

When appending the "git-archimport-id:" line to the end of log entries,
git-archimport would use two blank lines as a separator when there was no
body in the arch log (only a Summary: line), and zero blank lines when there
was a body (making it hard to see the break between the actual log message
and the git-archimport-id: line).

This patch makes git-archimport generate one blank line as a separator in all
cases.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-archimport.perl

@@ -595,7 +595,11 @@ sub process_patchset_fast {
     my $pid = open2(*READER, *WRITER,'git-commit-tree',$tree,@par)
         or die $!;
     print WRITER $ps->{summary},"\n\n";
-    print WRITER $ps->{message},"\n";
+
+    # only print message if it's not empty, to avoid a spurious blank line;
+    # also append an extra newline, so there's a blank line before the
+    # following "git-archimport-id:" line.
+    print WRITER $ps->{message},"\n\n" if ($ps->{message} ne "");
 
     # make it easy to backtrack and figure out which Arch revision this was:
     print WRITER 'git-archimport-id: ',$ps->{id},"\n";