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