Merge branch 'db/push-sign-if-asked'

Merge branch 'db/push-sign-if-asked'

The client side codepaths in "git push" have been cleaned up
and the user can request to perform an optional "signed push",
i.e. sign only when the other end accepts signed push.

* db/push-sign-if-asked:
  push: add a config option push.gpgSign for default signed pushes
  push: support signing pushes iff the server supports it
  builtin/send-pack.c: use parse_options API
  config.c: rename git_config_maybe_bool_text and export it as git_parse_maybe_bool
  transport: remove git_transport_options.push_cert
  gitremote-helpers.txt: document pushcert option
  Documentation/git-send-pack.txt: document --signed
  Documentation/git-send-pack.txt: wrap long synopsis line
  Documentation/git-push.txt: document when --signed may fail