add read_author_script() to libgit

Add read_author_script() to sequencer.c based on the implementation in
builtin/am.c and update read_am_author_script() to use
read_author_script(). The sequencer code that reads the author script
will be updated in the next commit.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.h

@@ -114,4 +114,7 @@ void commit_post_rewrite(const struct commit *current_head,
 #define SUMMARY_SHOW_AUTHOR_DATE (1 << 1)
 void print_commit_summary(const char *prefix, const struct object_id *oid,
 			  unsigned int flags);
+
+int read_author_script(const char *path, char **name, char **email, char **date,
+		       int allow_missing);
 #endif