codereview: fix clpatch with empty diffs

Avoid passing the placeholder diff to hgpatch, so that
clpatch-ing an empty diff grabs the metadata and warns
about it being empty, rather than failing with a
hard-to-debug problem ("mkdir: no such file or dir",
no metadata, etc).

R=rsc
CC=golang-dev
https://golang.org/cl/4172060
1 file changed