Merge branch 'jc/sign-buffer-failure-propagation-fix'

A failed "git tag -s" did not necessarily result in an error
depending on the crypto backend, which has been corrected.

* jc/sign-buffer-failure-propagation-fix:
  ssh signing: signal an error with a negative return value
  tag: fix sign_buffer() call to create a signed tag

diff --cc builtin/tag.c
Simple merge

diff --cc gpg-interface.c
Simple merge