cherry-pick: don't forget -s on failure

In case 'git cherry-pick -s <commit>' failed, the user had to use 'git
commit -s' (i.e. state the -s option again), which is easy to forget
about.  Instead, write the signed-off-by line early, so plain 'git
commit' will have the same result.

Also update 'git commit -s', so that in case there is already a relevant
Signed-off-by line before the Conflicts: line, it won't add one more at
the end of the message. If there is no such line, then add it before the
the Conflicts: line.

Signed-off-by: Miklos Vajna <vmiklos@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.h

@@ -49,4 +49,8 @@ extern void remove_sequencer_state(void);
 
 int sequencer_pick_revisions(struct replay_opts *opts);
 
+extern const char sign_off_header[];
+
+void append_signoff(struct strbuf *msgbuf, int ignore_footer);
+
 #endif