add.c: extract new die_if_path_beyond_symlink() for reuse

This will be reused by a new git check-ignore command.

Also document validate_pathspec().

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pathspec.h

@@ -4,5 +4,6 @@
 extern char *find_pathspecs_matching_against_index(const char **pathspec);
 extern void add_pathspec_matches_against_index(const char **pathspec, char *seen, int specs);
 extern const char *check_path_for_gitlink(const char *path);
+extern void die_if_path_beyond_symlink(const char *path, const char *prefix);
 
 #endif /* PATHSPEC_H */