If a file pattern is given and matches files that look
like they need to be hg added or hg removed, offer to do so.

If a file pattern is given and matches files in another CL, warn.

If a file pattern doesn't match anything, point that out.

Vet first line of CL description.

Fixes #972.

R=adg, niemeyer
CC=bradfitzgo, golang-dev
https://golang.org/cl/4099042