fast-import: add 'abort-if-invalid' mode to '--signed-commits=<mode>'

fast-import: add 'abort-if-invalid' mode to '--signed-commits=<mode>'

The '--signed-commits=<mode>' option for git-fast-import(1) configures
how signed commits are handled when encountered. In cases where an
invalid commit signature is encountered, a user may wish to abort the
operation entirely. Introduce an 'abort-if-invalid' mode to do so.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>