gpg: Request keygrip of key to add via command interface
* g10/keygen.c (ask_algo): Request keygrip via cpr_get. * doc/help.txt (gpg.keygen.keygrip): New help text.
This change makes it possible to add an existing (sub)key to
another key via the status/command interface.
(cherry picked from commit 19b1a28621c614b81f596e363b1ce49dd9fae115)
- GnuPG-bug-id: T5771