cmp(1) on FreeBSD requires two file arguments.  grep -P on Linux (at least
Ubuntu 12.04) is described in the man page as "This is highly
experimental" and doesn't seem to work. On FreeBSD the man page states
"This option  is not supported in FreeBSD."  Needed this to work while
debugging some funky behavior of 'Import' in my local vim setup.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/7675043