Merge branch 'sg/gpg-sig'

Teach "merge/pull" to optionally verify and reject commits that are
not signed properly.

* sg/gpg-sig:
pretty printing: extend %G? to include 'N' and 'U'
merge/pull Check for untrusted good GPG signatures
merge/pull: verify GPG signatures of commits being merged
commit.c/GPG signature verification: Also look at the first GPG status line
Move commit GPG signature verification to commit.c

diff --cc Documentation/merge-options.txt
Simple merge

diff --cc builtin/merge.c
Simple merge