send-pack: avoid redundant "pack-objects died with strange error"

Saying "pack-objects died with strange error" after "pack-objects died
of signal 13" seems kind of redundant.  The latter message was
introduced when the run-command API changed to report abnormal exits
on behalf of the caller (v1.6.5-rc0~86^2~5, 2009-07-04).

Similarly, after a controlled pack-objects failure (detectable as a
normal exit with nonzero status), a "died with strange error" message
would be redundant next to the message from pack-objects itself.

So leave off the "strange error" messages.

The result should look something like this:

	$ git push sf master
	Counting objects: 21542, done.
	Compressing objects: 100% (4179/4179), done.
	fatal: Unable to create temporary file: Permission denied
	error: pack-objects died of signal 13
	error: failed to push some refs to 'ssh://sf.net/gitroot/project/project'
	$

Or in the "controlled exit" case (contrived example):

	[...]
	fatal: delta size changed
	error: failed to push some refs to 'ssh://example.com/foo/bar'
	$

Improved-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/send-pack.c

@@ -101,7 +101,7 @@ static int pack_objects(int fd, struct ref *refs, struct extra_have_objects *ext
 	}
 
 	if (finish_command(&po))
-		return error("pack-objects died with strange error");
+		return -1;
 	return 0;
 }