Merge branch 'pw/add-p-single'

"git add -p" used to offer "/" (look for a matching hunk) as a
choice, even there was only one hunk, which has been corrected.
Also the single-key help is now given only for keys that are
enabled (e.g. help for '/' won't be shown when there is only one
hunk).

* pw/add-p-single:
  add -p: improve error messages
  add -p: only bind search key if there's more than one hunk
  add -p: only display help for active keys